[Thanks to Jimmy](I posted part of this way back in 2006, but only the middle, and a poorer scan. The work makes a tonne more sense with all three parts.)
Hieronymus Bosch's colossal painting The Garden of Earthly Delights (wiki).
Really big size scan here.
I think this is a monumental piece of art. It's beautiful, it's hugely complex but hangs together, it's funny, it's sardonic, it's hellish...
Gawd, it must take a special kind of mind to go through making a work of that size.
For my own desktop, I have broken it up into four parts: lower middle, higher middle, left side recomposed to landscape (bottom put on the right), and same with the right side, Hell. (Of course I also have the full original. It's a bit of sacrilege to break up a painting like that, but even with very large screens, you just can't really see what's on this enormous painting otherwise. (If you want them, I have zipped the parts into a zip file.)