I use GIMP too, and I get the same amount of colors that I used. Are you taking screenshots of these scaled without zscreen to keep those shots as PNGs? If you do straight screenshots, it saves it on the clipboard as a .jpeg or something.
Another guess: did you rescale your art? If answer is "yes", it could be an answer. It depends of settings for rescale (interpolation), try some different modes. But better DO NOT enlarge your art - here on PJ is usefull options for rescaling images, so people rather to see your art in original scale. Or, if you think it's too small, use double-sized images. But first check the mode of interpolation.
GIMP is OK, but something wrong with settings. It mixing colors, though shouldn't do it.
Try to redraw some small pieces in GrafX2 (it's absolutely freeware, and multi-platform):
http://code.google.com/p/grafx2/
And again too many colors (721! Wow!) What software do you use?
BTW: You have wonderful art in your blog!