Opera5.02Win and Mac has a really odd issue with the fixed background image. It shows up on pages long enough to scroll when you use a middle or bottom position. If your page is long enough, the image disappears. But not really. Use the zoom function to shrink the page. See? Opera places a bottom position image at the bottom of the page, not the window. And then keeps it that distance away from the top of your window as you scroll down, so you can never reach it. Nice one. Detailed explanation here.
Plus it interprets 'center center' as 'top center', thus forcing us to use '50% 50%' or a single 'center'.
Oh, and Netscape6.01Win slips all right side fixed bg images under the scroll bar. And so puts centered fixed bg images right a bit. So center in Netscape is different for bg images and for boxes. I guess Netscape didn't want to feel left out. Watch out if alignment is pixel critical in your web pages.
Click around the samples to check.