Be careful when opening and saving JPEG images. Repeated compression will cause a noticeable degradation of quality. The rule of thumb is that you do all of your image editing, then compress only when you're finished and ready to include the image in your Web document. Always save an uncompressed so that if you need to do further editing, you won't have to re-compress an already compressed image. JPEG compression can be applied in different degrees.
Image quality varies inversely with compression ratios.
The more you compress the more quality suffers. JPEG works directly on 24 bit color images, and therefore is immune from the compromises of 256 color palettes. These behave similarly to interlaced GIFs in that the image becomes progressively more visible as it downloads. It's also the only way to go for animations and transparency effects. GIF is used for most non-photographic images.