Choose the desired format.
In general, images that have been created on a computer, including logos, banners, line art, text, and any graphic with large areas of a single color and sharp detail should be saved in GIF or PNG-8 format (Figure 5.5)
Figure 5.5. Here is part of the Save for Web box that shows the original image (upper left) and three possible compressed versions. This image has a lot of flat color, as well as text, which should be kept sharp. Note how the GIF format (lower left), compresses the image the best, to just over 8K. JPEG at high quality is huge. At medium quality, it's big and ugly.
Images with continuous tones, like photographs, should be saved in either JPEG or PNG-24 format (Figure 5.6)
Figure 5.6. The GIF compression leaves banding in the photograph (bottom left). The two JPEG options on the right are smaller and better quality.