The easiest way to avoid copyright problems with a collage is to use only material that is your own copyright (such as your own photographs), copyright free material, or images from sources which clearly say they can be used, such as these reference photos or photos in Flickr licensed with a derivatives/commercial use Creative Commons license.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.

