PNG files are best suited for almost any type of web graphic due to their flexibility and small file size; however, the display of PNG images is only partially supported in Microsoft Internet Explorer (4.0 and later browsers) and Netscape Navigator (4.04 and later browsers). So unless you are designing for a specific target audience using a browser that supports the PNG format, use GIFs or JPEGs for broader appeal.

GIF (Graphic Interchange Format) files use a maximum of 256 colors, and are best for displaying noncontinuous-tone images or those with large areas of flat colors, such as navigation bars, buttons, icons, logos, or other images with uniform colors and tones.

JPEG (Joint Photographic Experts Group) file format is the superior format for photographic or continuous-tone images, because JPEG files can contain millions of colors. As the quality of a JPEG file increases, so does the file size and the file download time. You can often strike a good balance between the quality of the image and the file size by compressing a JPEG file.