At the minute I don't have one. I feel the pictures can look very tacky. 3D backgrounds look great however they are expensive and take up valuable space in the tank. I would like to get something at some point but undecided yet.
I have noticed my favorite tanks have a dark color background. My father-in-law's tank has a busy background, along with brightly colored decoration. With a black/blue background you're eyes go straight for the fish. Just my personal taste. To each their own. Mines dark blue.