Can i use iPhoto to trim my photos?
In contrast to Lizzan, I suggest you do it with iPhoto. So the answer is yes, you can do it with iPhoto. The reason why I suggest it is because iPhoto never destroys your image! It keeps the original image as it is and saves your changes to a separate file. You can always restore your original image. I don't know the OS you're using but prior to (I think) Lion there is no Version feature so returning to your original image is impossible.  

So here's how you do it: http://support.apple.com/kb/VI191  

You could crop the region you like and then save it as a new image. Then go back to your original and take the next region...