Learn GIMP - you won't regret it. I'm a GIMP expert, and I've got 10 tips for getting productive faster with this great Photoshop alternative.
In the Gimp, you should be able to simply use the elliptical selection tool (E) to select the area you want, and then either copy and paste into another image, or Select->Invert (Ctrl+I) and then Edit ...
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