photo with certain color as background

I want to make the texture for some trees. The solution is a very common one, by using Alpha. But question is: how can i take a photo with the background color as i want(in reality the background can never be one solid color) or is there a tool that can do something to the photo in a easy an clever way as we want or is there a place where i can get such ready made photo?
Thanks

photoshop->image->extract