Lossy compression can eat away at the quality of your image. Photoshop recompresses a JPEG image every time you save it in JPEG format. During a single edit session, this compression won't hurt because JPEG works from the on-screen version. But if you close, reopen, and resave the image in JPEG format, degradation occurs. You may not see the results right away, but you will over time. Leave your image in either TIFF or native Photoshop file formats while editing. Then, when you're completely done editing and you need to compress the image, save the file as a JPEG at a high to maximum quality setting.