I was looking for something mainly to add borders and writing. I didn't see where you do that with Gimp
I'm new to Mac also. It seems that to install most programs is Mac it's just a matter of copying the program file to the "Applications" folder. To uninstall it you just delete it from the "Applications" folder.
In the case of Photoshop Elements you could download the 30 day free trial from Adobe.com. It actually has an install program that installs the program and then to uninstall it you need to run an uninstaller program that comes with it. You dont need to copy and delete from the Applications folder.
To add borders and text you could use Google Picasa. It is a very nice photo manager that has basic editing functions like text and borders. It is a free download from here:
Picasa download in progress
I think iPhoto can also do text and borders but I'm not sure.
iPhoto and Picas are both photo managers so they may be in conflict if you use them both. iPhoto likes to take your photos and hide the folders where it puts them. Picasa likes your photos to be in the /pictures folder. Picasa makes a duplicate of all of your photos in iPhoto to /picturs folder and can get kinda messy. I removed iPhoto from my machine and only use Picasa.
Hope this helps and is not too confusing....