Honestly, think about the site you're designing for, and it's theme. Otherwise, you can't really have an extensive design for them, and make it detailed.
For example, if you're creating a header image for a site that revolves around rating pictures, you could include a vector of a camera, then have different Polaroids spread out with pictures and "5/10" around them. Not only will it be eye candy, but it will be great from a guest's point of view. Not only would they know what the site is about, but they'd be lured into joining because of the great design.