I am trying to find a photo shop program that allows cropping by hand so I can crop a head and put in another of my pictures

i used to make greeting cards by cropping faces and putting them on funny pictures that i would download from the web. i recently purchased a macpro and was disspointed to learn that i cannot cut out a face by hand on iphoto. Do you know of any ..other program that I can use with my macpro to accomplish this. Thank you in advance for any help you can give to me.