I've downloaded many very simply images (in the public domain, of course) that were surprisingly JPGs, rather than PNGs or GIFs. I assume they were created by people who didn't know what they were doing, as the file sizes are often ridiculously big.
For example, I recently downloaded a map that consists of just three colors - the background color, black (text and national borders) and the color red indicating an animal's distribution. The file size should be less than half than what it is.
But when I copy it into a new Photoshop document, then try and save it as a GIF or PNG, the file size is still big, sometimes even bigger.
So, is there a way to convert such images to PNG/GIF and reduce the file size? Though I've used Photoshop for a long time, I've never really mastered the details, so there may be a simple solution(s) I simply haven't stumbled over yet.
Incidentally, I'm using CS3.