However, for simplicity, why not just load op the photos you want onto a CF card (or SD or whatever) and have the frame display the pictures directly from the card? When you want to add new pictures just take the card out upload from your Mac to the card and re-insert into the frame.
I concur with this statement, however, whichever way you choose to display your images, I can recommend the Kodak range of frames. I own a 10" Kodak and have had experience with other brands that were nowhere near as user friendly (multiple select, better menu's, automatic resizing for internal memory etc). Also the Kodak has a matte screen that you can view from any angle without getting reflections or contrast changes, (if you choose another brand, I would make sure it also has this before buying, otherwise believe me you will want to throw it in the bin).
Oh yes....and it is WiFi capable if you want to display images from you computer.