Thank you all for replies.
I think I probably expressed my problem in a bad way.
Let's say I have a background for my html (image A) and a background for my body (image B) wich is 800px wide. I want the edges of B to disappear, gradually revealing A, regardless of monitor resolution or body position. So I can't use photoshop or such because B is not always on the same position relatively to A.
Is this possible?