Customize Background

I have a question...I know it might be a simple one, but I need some help.

For my community...I changed the background colors and all - though now I can't place my lovely header/banner up on the top. I know many users that have wonderful headers/banner that stay on the top of their page - but I'm guessing I'm doing something wrong cause half of it gets cutoff.

If there is anyone who can help me - it's very much appreciated.

Cause the last time I had one - a friend of mine did it for me. So any help would be great!!