Hi,
Not sure about Iphoto will do the task. If you want to see if there's any Mac freewares that can do image editings, you can try looking up the Pure Mac web site ( http://www.pure-mac.com/ ) and go to Graphics under Multimedia. Mac version's GIMP is listed there. Also, you can try the free version of ArtRage at http://www.ambientdesign.com/artrage.html.
Hope this will help.
vince