The GIF89a format supports transparency, interlacing, and multi-blocks. Although the GIF format does not distort colors, it is an 8-bit format. If the background color in your graphic must match the background color of your web page, use a GIF image. Because of the compression algorithm, a JPEG image may not be true to color. Since the algorithm is based on what the eye can see, however, the decompressed image is generally hard to distinguish from the original except at high magnification.
Artifacts are most likely to be visible in large areas of the same color.
Since JPEG compression can be applied to varying degrees, the more compression, the lower quality the result. At least with JPEG images, you do not have to worry about browser safe colors. It is impossible to do more than mention layout in the present crash course in web site design. Whichever style you choose, be consistent . There is nothing more annoying than having the layout change from page to page for no apparent reason.