The image needs to replace the background image, not lay on top of it. You can't ostrisize firefox users, because they are around 10% of the internet users. Here is a picture of what I am talking about:
See how it covers important buttons: my controls, view new posts, etc.
Also see where I put the *. THat is where the image should be placed. Find the tile_back.gif located in /forums/style_images/1/ and rename it to old_tile_back.gif. Change the name of /newbanner.jpg to tile_back.gif. Or go into your css and change the name of the tile_back.gif file to newbanner.jpg.
Contact me if you have any questions. I do design too.