Photoshop has such mindshare in the work world. Any time people think professional photo editing, they think Photoshop. And it is a great tool. But it is way overkill for what most marcom folks need.
I used the awesome web site AlternativeTo to find Mac alternatives to Photoshop. Pixelmator was high on the list, and for good reason. It has some amazing and easy-to-use photo editing features. Check out the 2-minute video on this page for a pretty fantastic overview of what it can do. The repair tool (a.k.a. healing brush) alone is worth the cost of the app.
The crop tool is also very useful. It can constrain to any aspect ratio and shows guides for the rule of thirds.
I cannot recommend this app highly enough. I am standardizing our whole marcom team on it.