Your best bet is to save your original graphic in TIFF or Photoshop format, and then Save As to JPEG format with different amounts of compression, and compare and contrast image quality, as well as file size. Baseline Optimized JPEG format produces better color and a somewhat smaller file size but is not supported by all Web browsers. Progressive format creates JPEGs that display gradually as they are downloaded to the Web browser, much like interlaced GIFs. Progressive JPEGs are larger in file size and are not supported by all Web browsers. It is important to note that saving a graphic to JPEG format with compression should be a last step.
Compression effects are cumulative.
If, for example, you save a graphic in JPEG format with a compression of 7, and later decide that you want to save it with a compression of 3, DON'T recompress the graphic that already has a compression of 7. Go back to your original, and save it with a compression of 3. Otherwise, your image will suffer the consequences.