Funny you should mention that.. This was, surprisingly, one of the most discussed topics internally when developing the app. The background image changed almost daily over a 3 month period (along w/ 2-3 complete redesigns). We wanted something that conveyed warmth, food, friendliness and was aesthetically pleasing. The process was really just lots of trial and error (and photoshop), but we're happy with how it turned out. We think it fits well with the food/restaurant theme - almost like you're sitting down at a table with friends about to enjoy some delicious food.
Now the website has to play catch up!