Depends on the image. Sometimes you can slice them up into just the most pertinent parts, and use background colors to set the solid parts. But if its a picture with a lot of detail, you probably can't do that, in which case it's most likely not most conducive image to web design. First, as you said, it will take a long time to load, and second, having a set background will often make it so users cannot expand the site (increase text size) without the site falling apart and looking bad.