Save your working copies as .bmp or some other format that doesn't compress them at all. Never ever work in .jpg. Every time you save you lose more quality because you're re-compressing it over and over. .gif isn't lossy, but it's limited to 256 colors or so so it's not that useful. .png is OK, but .bmp is probably about the safest you can get. .bmp creates huge files though. Work in .bmp, and convert the final image to .jpg or .whatever.