Looking at data from our clients, we realised that a very small percentage of users were uploading their own profile pictures. Further investigation revealed that many aren't even able to get imagery on to their computers so there was little hope of them uploading something to our site. This meant that most of our users were just seeing dark silhouettes.
We researched a number of alternatives including using a library of images and combining multiple layer. Most of these were not practicable within the time allowed. In the end we settled on enabling users to choose their initials and the background colour.
What you see below is interface I designed for the user.