chans = ColorSeparate[img];
mask = Binarize@
Rasterize[
Graphics[Disk[{0, 0}, .5], PlotRange -> {{-1, 1}, {-1, 1}}]];
ImageCrop@ColorCombine[ImageAdd[mask, #] & /@ chans];
SetAlphaChannel[ImageCrop@ColorCombine[ImageAdd[mask, #] & /@ chans],
ImageCrop@ColorNegate@mask]