I've just made an "eye-candy" patch to putty - making it fake-transparent (like transparent xterms).
It just loads putty.bmp from the same directory as putty.exe is in, and uses it as background,
positioning it as the window moves.
I'm just using the darker version of my windows desktop as putty background (see the screenshot)
Background could be turned on/off from the system menu.
Background is only applied to black background color.
Have fun with it! :))
screenshot
Download:
patched putty.exe
source

Unfortunately I don't have time to keep this project up to date with putty releases, but Markus Koskinen provides a more uptodate version on his webpage:
http://www.cs.helsinki.fi/u/mxkoskin/