PicMonkey is an online image editor built right into Shift. It's amazingly flexible - you can do everything from resizing and cropping your images, to adding graphical borders on your images, to adding text and image effects.
You can access PicMonkey by going to
Dashboard > File Manager > Click on the image you want to edit (once it's uploaded) and click 'Edit'
It's all pretty straight forward - just follow the prompts.