User blog comment:Jkphantom9/Beaconship - Vote For Your Favorite RWBY Characters!!/@comment-1292715-20151121072722/@comment-410526-20151122064608

For a Mac, hit Shift + Command + 3 and it should save a file called "Screenshot (date+/time)" on your Desktop. You can then crop that in Preview.

Or hit Shift + Command + 4 and it'll bring up a special cursor where you can capture just a portion of your screen (which will also be saved to a file called "Screenshot (date/time)" on your Desktop).