Most photo editing type of programs will work.
do you have one already?

If you have MSWord there should be a picture or photo editing section??
I used it for adding a photo, cropped it to oval also, for my parents 50th anniversary invites. It was a bit confusing tho.

a free one - pretty easy
http://picasa.google.com/

or a list of free ones here-
http://www.download.com/3150-2192_4-...49&dlsize=&ca=