We want to give my Dad a picture of himself and my Mum that is generated from a mosaic of family pictures from my sisters and me. I've experimented with a couple of tools for doing this but there seems to be a couple of limitations:

1) Pictures are resized to squares and I'd like to keep the original ratio
2) There is no guarantee that all pictures are used - I'd like to try and get an even distribution across the pictures we are using if we can

Any thoughts or recommendations on tools I might try that address these shortcomings?