Is there a free photo editor for mac that can for example put a human head on a dog? Like an editor that let's me draw around an object and move it where I want to in the same picture or a different one. The grab feature in preview only lets me draw a square. I want to be able to draw the shape I want. Thanks

EDIT: I have gimp but I'm not quite sure what to do. If anyone knows how to do what I want in gimp, please explain how