I agree that you do need a background, as others have mentioned.
If you have live plants in the tank, I wold not get any "photo" backgrounds with plants; these never look good. Fish often do not look good against colourful backgrounds either. The best is plain black construction paper or card that was mentioned in post #3. I use black construction paper on all my tanks. It increases the perception of depth in tyhe aquarium because with decor/plants/fish it is basically invisible. And everything in the aquarium looks better, regardless, as it is not competing with the background.
You want nothing shiny because it reflects light and is very noticeable. And it can disturb fish. Black construction paper works best for this.