Pixlr is a free, online photo editor that is a good alternative to software such as Photoshop. It can be used on the web and there is an app for iOS and Android. There is also a desktop version that can be downloaded for Mac or PC. This software would be ideal to use for editing your own photos or for recommending to students if photo editing was part of their course.
Pixlr is an unsupported tool so be sure to note our guidance on the usage of unsupported tools before use.