It depends on the image. If you are just trying to turn an off-white background to true white, you can probably get the job done with everything from levels, to selective color adjustments, to the Dodge tool.
If you are trying to isolate a subject from a complex background, I have to say the only way to do this is with the Pen tool. I know this is in CS2, I have no idea about Elements. It can be very time consuming if the subject does not have nice, straight edges.
Zoom in 200% to 300%, and just start drawing around the object with the pen tool. It can be tedious, and it takes some getting used to. I'm not very good at it frankly, and iStock usually laughs off my attempts.