I've been using the same set of photographic portraits in my mockups for a long time, and I'm looking for a more extensive set of square, male and female avatars/photos (portraits).
What are good sets to use?
I've noted this set before. But it's worth repeating.
Automatic UI bonus points if your avatars include luchadores.
Who doesn't like extra credit? So what are you waiting for: Head to IconFinder to get the set.
There's a lot of avatar sets available online for free. Here's one example. Just use the shapes and re-style them for the needs of your design.
Most of the sets can be very easily found through google, the real art though is in getting something that fits the layout. Usually you'll find that it's best to obtain an AI or PSD file and change colors / add gradients so that it'd be closer to the design and color pallet you use.