Today, that's laughable. Now don't get me wrong- I once rode high for a year after someone paid $600 for one of my own canvases- but even when adjusted for inflation $2.4 million for a great painting is a pittance. And a Velázquez to boot!
What makes determining the most valuable painting in the world difficult is that most extremely high-end paintings (such as the Old Masters' or icons such as Guernica) are not for sale. In 1962 The Mona Lisa was insured for $100 million, which with inflation figured in makes it worth about $670 million today. But no matter what you're willing to pay for it, it will probably never be yours.
But here are the ten most expensive paintings ever purchased by individuals or foundations at auctions or private sales- along with a brief explanation why each might have sold for so much. One thing that is interesting to note is that, although all prices are adjusted for inflation, nine of the ten were purchased in the past twenty years, with four (including all of the top three) purchased in 2006. This may insinuate that our appetites for spending have become a little less satiable in recent years (hey, look at the credit crisis!). I will also add that these figures are truthful as of September 16, 2009; while the number one spot on our list has retained its title for nearly three years, its predecessor kept it for only five months.

Why So Much? Well, it's a self-portrait by Van Gogh. And he doesn't have a beard. Come ON now.
In my personal opinion, Van Gogh's work is somewhat overrated. But this is one artist whose legend cannot be separated from his work. Whom else do you think of when you correlate the words "brilliant" and "tortured"? He went mad from eating chrome yellow paint. He cut off his earlobe (not his entire ear) and mailed it to a prostitute whom he was in love with (although he was also a closet homosexual... hmmm). He painted nearly eight hundred paintings in his lifetime and sold only one. He offed himself at the age of 37- about ten years before he would become one of the most influential artists of his time. Oh, what a sad man. And sad men are the ones who are often taken the most seriously as artists.
Van Gogh's self-portraits are rare- while he painted many, only a fraction survive- so they command a huge price. This one, however, has garnered the highest sum.
9. Vincent Van Gogh, Portrait of Joseph Roulin. 1889. Sold 1989, $100.8 million.

Why So Much? Generally a loner who didn't have very many friends, Van Gogh was very close to the Roulin family when he lived in Paris Arles (thank you Nico), painting dozens of portraits of Joseph (usually in his postmaster uniform), his wife, and their three children. This particular one was not among the most recognizable nor the most impressive (although that wallpaper sure is fancy). So let's just file this one under "who the hell knows."
8. Pablo Picasso, Dora Maar au Chat. 1941. Sold 2006, $101.8 million.

Why So Much? Picasso holds a Guinness Book record as the most prolific artist of all time; over his eight decade career he produced over a million works. Before you accuse me of shitting you, at least half of these were prints from editions; nonetheless, he was still known to produce as many as six paintings a day. So why would a painting by someone who obviously has so much work floating around be worth this much? Well, influence wise Picasso was the greatest artist of the twentieth century, and this is one of the paintings that make us see why.
It's a rare three quarter pose of Dora Maar (his most mysterious mistress; I have discussed her before) and it has a dramatic, almost sculptural quality in its line and shape. Even by Picasso's standards, this is simply a good painting. If Les Demoiselles d'Avignon showed us everything Cubism could be, this painting embodied what it became. (I'm sorry- but I like it.)
So who owns it? No one knows for sure- although rumor has it that it's Georgian mining magnate Boris Ivanishvili, who sold a bank that he owned in Moscow for about half a billion dollars a week before the auction and has since then sort of kept to himself.
7. Vincent Van Gogh, Irises. 1889. Sold 1987, $102.3 million.

Why So Much? It's quite possibly the most famous painting in the top ten; there's a good possibility that you've owned an umbrella or ceramic mug printed with it. And Van Gogh painted it while in an asylum, for crying out loud. He referred to this painting as "the lightning conductor for my illness," meaning that painting it was all that kept him from going insane.
6. Pablo Picasso, Garçon à la Pipe. 1905. Sold 2004, $118.9 million.

Why So Much? This was one of the finest example of Picasso's Rose Period, which immediately preceded his invention of Cubism; but its sale at such a high price has still been a quandary to many. Said Picasso expert Pepe Karmel, "I'm stunned that a pleasant, minor painting could command a price appropriate to a real masterwork by Picasso. This just shows how much the marketplace is divorced from the true values of art." Uh... damn.
5. Pierre-Auguste Renoir, Bal du moulin de la Galette. 1876. Sold 1990, $128.8 million.

Why So Much? The oldest painting on the list, this rather well known and optimistic snapshot of Impressionist life was sold to Ryoei Saito, the honorary chairman of Japan's Daishowa Paper. And he must have been enamored with it- he announced that when he died he planned on being cremated along with it. After an enormous public outcry to this statement he claimed he was only joking. That was probably bullshit, but hopefully we'll never know, as when Saito experienced a little financial difficulty he had to hand it over to a Swiss bank as collateral on a loan.
4. Vincent Van Gogh, Portrait of Dr. Gachet. 1890. Sold 1990, $136.1 million.

Why So Much? What do we know about the subject of the painting? His name was Dr. Paul Gachet, he was a Parisian physician who worked with mental patients (including Van Gogh), and in Van Gogh's opinion he really wasn't that good at what he did. In fact, Vinnie once said of his doctor, "[He's] sicker than I am, I think, or shall we say just as much." It is interesting that Van Gogh painted his doctor in the same style as Delacroix's paintings of Torquato Tasso in the madhouse.
This painting passed hands with about eight owners, including Herman Goering (of course, in his case it was more along the lines of theft). Ultimately, this was another purchase by Ryoei Saito, which was also destined to join him in the crematorium before his creditors stepped in.
3. Gustav Klimt, Portrait of Adele Bloch-Bauer I. 1907. Sold 2006, $144.4 million.

Why So Much? Now even for Klimt that is a lot of damn gold. He spent three years working on this piece, putting it amongst the finest of his oeuvre without a doubt. The wife of a wealthy Jewish industrialist, Adele left this painting and others that she had commissioned to the Austrian State Gallery in her will. They never made it there though; after the Nazi occupation of Austria, her family was forced to flee to Switzerland and their art collection was looted. After the war, a huge legal battle began between the Austrian government and Adele's nephews and nieces, including Maria Altmann. Many years and millions of dollars in court fees later, the painting was Altmann's, and she was free to sell it to cosmetics tycoon Ronald Lauder for an obscene amount of money.
Lauder bought it to display in his Neue Gallerie, though it made its debut in his ownership at MoMA, where visitors were charged $50 for tickets just to see it. It may sound exorbitant just to see one painting, but hey, I'd fork it out. Well, as long as I got to touch it. Look at all that gold. It's begging for my greasy fingerprints...
2. Willem de Kooning, Woman III. 1953. Sold 2006, $147 million.

Why So Much? It's super abstract, mildly misogynist, revolutionary for its time, and your kid could probably do it, right? There was a friendly battle going on for many years between de Kooning and Jasper Johns over who was going to be the world's most expensive living artist. In 1997 de Kooning had to bow out when he ceased to be a living artist.
Formerly in the Tehran Museum of Contemporary Art in Iran, it was removed after the revolution; it wasn't to Khomeini's taste. It somehow ended up in David Geffen's ownership, who went on to sold it to hedge fund manager Steven A. Cohen, who's also the owner of history's most expensive dead shark (you know the one). Since Cohen apparently has little actual taste, his motive in spending so much on this piece was probably to show the world how rich he is.
And now for the most expensive painting of all time... by the artist who, according to de Kooning, "has broken the ice for us."
Ready?
Drumroll, please...
1. Jackson Pollock, No. 5, 1948. 1948. Sold 2006, $149.6 million.

And after all, he was the one to break the ice.