I use digital elements in my cards quite a bit, and I'm also on a mac
I use photoshop elements 6, which I love. The newest version, elements 9, sells for about $70 but there are some rebates going on now that you can get $20 off.
You can also use Word.
Here's what you do in Word:
open a new document, then from the dropdown
insert > picture > from file
find what you want and put it in the page
then if you want to resize it,
format > picture > size
let us know if you have aqny other questions, and good luck!