The ten top painters who do best among the poster-buying public relative to their more moderate historical prominence (i.e., their influence on subsequent artists) are:
Henri de Toulouse-Lautrec
Vincent Van Gogh
These are definitely not unimportant figures in the history of art -- they're just even more popular now than they were influential then.
Basically, to sell a lot of posters in the 21st Century, you will have wanted to have been in Paris in the late 19th Century.
The Impressionist Claude Monet absolutely dominates poster inventories, with the Expressionist Van Gogh second and the Impressionist Renoir third.
Henri Rousseau was an obscure retiree who painted charmingly childish fantasy jungle scenes cobbled together from the Paris zoo and horticulture exhibit. He's famous today because the cool kids on the early 20th Century Paris art scene, such as Picasso, Brancusi, and Apollinaire, adopted him as a sort of mascot. Who knows how many other primitive painters could have made it big if they happened to be in Paris in 1905? Well, it's good that he got lucky because his pictures make you happy.
The ten painters who sell the fewest posters relative to their historical importance are:
Pol de Limbourg
Antonio del Pollaiolo
Giorgio de Chirico
Hugo van der Goes
A few comments: Masaccio was the first painter known to use perspective (which was perhaps invented by his friend Brunelleschi). That makes him a very big name in the history books. But he died young, so we don't have many examples of his work. And his colors aren't terribly vivid because the Florentines didn't yet have the Norwegian invention of oil paint, plus his frescoes have faded somewhat over the last 600 years. So, posters of his work wouldn't do much to liven up your living room with a splash of color, the way a Monet poster does.
The popularity of Impressionist painting posters is related to the fact that posters of paintings compete with posters of photographs. If you want a clear image of something to hang on your wall, you'll probably buy a poster of a photo, not a painting by, say, Poussin. And if you want to look at an image from Roman history, you'll probably go to a Ridley Scott movie rather than buy a Poussin poster.
The Impressionists were the first painters to figure out that photographers were starting to compete with them and they'd better do something that played to painting's strengths over black-and-white photography: color and mood.
They were very successful at creating attractive colorful canvases. By being the first movers, they were able to grab the lowest-hanging fruit of a strategy: Go someplace nice-looking when the sun is shining and splash a lot of color quickly on the canvas. Hey, this isn't rocket surgery.
Ever since, painters have been striving to distinguish themselves by inventing new alternative strategies to combat competition from photography, with ever diminishing returns.
Sculptors count in Murray's rankings--thus Michelangelo comes out #1--but a poster of a sculpture is kind of, well, flat. Thus, the Big 4 sculptors, Donatello, Michelangelo, Bernini, and Rodin, don't sell that many posters.
Reproducing sculpture well is vastly more difficult than reproducing a painting well. For example, the Florentines put a reproduction of Michelangelo's David outside when they moved the original inside to protect it from the elements. Presumably they put some effort into getting a good reproduction, and being Florentines they access to the best copyists, but the result is still ho-hum. When you see the reproduction, you say: "Hey, look, it's that big naked guy, you know, what's his name?" Then you go inside to see the original and say: "Wow! That's the greatest work of art in the entire world!"
I looked at a lot of famous paintings over six weeks in Europe in 1980, but it was mostly not too different from looking at them in my Janson's History of Art textbook. The "David," though, is in another dimension altogether.
Raphael: Since AllPosters.com is in English, I presume it's aiming at an American audience. American tastes are influenced by what's in our museums, both directly and indirectly (art history textbooks tend to use examples from the writer's local museum -- that's why the Chicago Art Institute seems almost as good as the Louvre to somebody who took art history using the Chicagoan Janson's textbook).
There are very few Raphaels in America. The only one I've seen is a small but ineffably beautiful Madonna in the Norton Simon in Pasadena. Raphael died young and was recognized as a genius in his own lifetime, so his works were treasured from the start. The Pope would pay to have first dibs on Raphael's work. His greatest painting, The School of Athens, is on the Pope's apartment's wall, and he ain't selling.
In contrast, Monet painted a lot of canvases, and Americans were rich enough to buy up a fair number.