Choose a theme for your photo scene. (western, steam-punk, science fiction, etc.)
A good source of images to start with is Deviantart.com If you are looking for images on a transparent background, source for the object you want + png (For example: rock png or flower png).
You can find backgrounds by searching for backdrop, background, scene or giving it a more specific search like moonscape or barn. You can also do a google search for key words. Make sure to do your image search on Large Images