Kinda vague question. =O) I do most of my BG doctoring with Photoshop most of my postproduction too. Try as best you can to get pictures that line up at approximatly the right angle. Nothign like a render that looks like yer character is wearing skiboots. =O) Keeping the lighting color the same adds to realism too. Photoshop is good for that too. I took a daylight shot of a forest and just f***ed with brightness and color balance and made it look like a night shot. The bg of my AvP pic is one.