EasyFrame makes this task so simple and quick that you may find yourself putting a border on everything.
EasyFrame comes with a nice built-in collection of frames. But better yet, it lets you add frames you made and found on the web to your own custom sets.
What's New in Version 2.4.5
Bug Fix: Now works correctly on MacBook Pro with Retina Display.
What's New in Version 2.4.6
Added a Share button (feature requires Mountain Lion).