There are a lot of tools to build a collage available online. I've found all the ones I tried really compromised image quality substantially.
So I did this one using Photoshop!
Took some time, but I think it's worth it for the image quality.
After having completed this collage with Photoshop, I discovered Photo Collage Maker Pro from AMS Software and decided to give it a try. I have to say that the process with this software is so much quicker and the result is equally pleasing. So much so that I purchased a license after trying it out in trial mode.