Making JPEG backgrounds are a popular choice. Present your material favorably at your site. Light images look best against dark backgrounds and dark against light. Black and gray doesn't allow much contrast, black on white does. Surfers won't get headaches and suffer eye strain.
Light colors imprint our brain into thinking it's actually white.
Use black or another dark color if you have a lot of pics. Lossy-creates small files by eliminating image info. Lossless-compresses without effecting image. Browser support two formats. If you have flat-color graphics high in contrast use GIF's.