Jpgs save as a smaller file size because they compress the information, which is great for saving space on your HD, and in the old days (ha, not too many years ago really) nobody had as much space on their 'puters as they do now, so that was kind of necessary. The problem with compression, though, is any time you may make a change to a jpg and then re-save the file with said changes, you lose information, so if you make changes and keep saving, over time the file degrades to a point where it is very pixelated. For posting purposes, jpgs are fine, though I always save my files as pngs. Only jpgs, gifs, and pngs are allowed/acceptable file extensions here, and probably most other sites as well.