The Pixlr editor is recommended for users who have a strong interest in art or photography and want to advance their photo editing skills.  The app enables users to crop, re-size, rotate, adjust exposure, add filters and more to their photographs.  Pixlr enables users to edit their photos on mobile devices and offline. This interface looks a bit like a traditional photo editor (similar to Photoshop) and is available as a web app, mobile app or can be dowloaded to your computer.  The users can access these tools for free with a basic account but there are some advertisements. 

PicMonkey enables users to edit their photographs and create collages. Images can be quickly uploaded from a personal computer or site like Facebook, and cropped, touched up, enhanced by effects, and more. This tool is recommended to help students develop basic photo editing skills. The mobile app is free but there is a monthly fee ($4-$8) to use all features (including saving your photos) on the website. There is a seven day free trial period. Here are tutorials to learn more.