These are created from this previous OGA submission: http://www.opengameart.org/content/4-summoning-circles

I took the originals and gave them a nice glowing effect so they'll look more magical.  This should make them look good when targeting a spell effect, for example.  Included are all four xcf files at 1024x1024 size.  They can easily be altered in a few ways by simply toggling the visibility of the layers.