Amazing Circles instructions: http://brilliantdays.com/how-to-create-amazing-circles/

I also did some circles using Flaming Pear plugins. Have step by step instructions showing the image transformation for making circles for Eye of the Flower. Other circles done with FP as well as well and more Amazing Circles. This one is really gross... I find strong lines do well.

http://www.pbase.com/cits_4_pets/altered_reality has various circles and other manipulated images.