You probably wont find anything like that unless you rip it from another game. Real life images and photos are rarely taken (and even if tried, its very difficult to get them to work) in a way that allows easy importation to a game. Especially a side scroller.

You might want to try getting a 3d modeller, and getting free building models off the web and then rendering them from a side position.

-Geoff

If you don't have cash to pay for a 3D modeler, trueSpace1 has been released for free and is available from a number of UK magazine sites. If you can't find it, or decide you don't like it, POVRAY is a free render engine that may suit your purposes, and there are many front ends available, such as Moray, which are very capable modeleres and provide access to custom object shaders and post processing shaders.

I'm creating a 2D side scroller and I need a a background image of a city. Where could I find something like that?

3D rendering a background isn't that great of an idea, because you will end up with an image that has been skewed by the perspective, which won't tile well. You might be able to fix that by increasing the FOV of your camera. You could also try 3D rendering one piece at a time and putting the whole thing together.

However, if I were you, I would whip out Photoshop, or Paint Shop Pro if you don't have Photoshop and draw your background.

-Chlorine