It's not cropping when its not a perpendicular shape such as a square or rectangle.
What you need to do is make the parts you dont want to see transparent, select round the items you want to keep, invert selection, delete key. Then youre left with your subject image and a background with small grey and white squares in a checkerboard style, that means its transparent.
Save your image as a .PNG, nwo when you view it or upload it, it will jsut be your subject with no background.
Hope that helps.
IndyChav