If you are so inclined here is the tutorial (it is fun and addicting - I would be happy to see more people sharing theirs and not feel at all that you are copying :-)



http://brilliantdays.com/how-to-create-amazing-circles
You need to scroll down a bit. There are basically 4 steps
1. make your pictures a square (crop or size)
2. Polar coords one way
3. rotate 180
4. Polar coords the other