I've put together a simple plugin for viewing images in Winamp, called ImageAmp. It lets you add jpeg, png, gif, and bmp images to the playlist, showing them in the standard Winamp video window. (Animation is not supported though.. you'll simply get the first frame of an animated gif).
A couple simple options let you specify rotation, max display size, and display time. Other than that, the standard Winamp video window options (like 'Close video window on stop') apply as well.
The plugin is available here:
It should work on Windows XP and later, though I've only tried it on XP.
I'd like to have a couple people take it for a test run before I release a 1.0. If anyone tries it out, please let me know if there are any problems.
Full C++ source code is up as well, if anyone is interested.