In Gimp 2, use the magic wand. What you want to do to get a perfect heart shape to cut around is to create a new layer with your heart shape. Use the wand and select around your shape. It will select everything around the shape. Inverse your selection, and work with the layer with the image you are using, and you will be able to cut the heart shape selection of your image to paste. If you just want to cut around the heart, there's no need to inverse your selection so you can skip that.