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 ...
PCMag on MSN
GNU Image Manipulation Program (GIMP)
G NU Image Manipulation Program (GIMP) is the longtime open-source photo editing alternative to Adobe’s category-leading ...
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