JPEG is a lossy copressed file, which means when you save a picture as a JPEG the computer throws out some information in order to save space. If it's not throwing out too much you can't notice the missing information because it blends in well. If the picture is highly compressed, or if it's saved over itself over and over, the missing information becomes very obvious, especially near edges and in color gradients.
thanks man! I was wondering wtf was happening to all my galleries (all 24gb) One more thing the wiki doesn't make obvious -what format SHOULD i use in place of JPG since i edit them a lot? TIF is like 4mbs for 1 pic, and BMP is too obvious. Maybe some foreign HD format.