Many teachers and students can’t afford the hefty price tag that comes with Photoshop and the requisite Adobe Creative Suite. Pixlr is my favorite great web-based alternative to Photoshop. It is a high-end photo editing option that teachers and students can access right from their web-browser. The layout and features of Pixlr are very similar to Photoshop, most importantly it has the same "layer" function.
Fotor is another web-based photo app that is easy-to-use and allows the user to create interesting collages, edit photos and design various print and electronic publications. Fotor is three web apps in one and it allows the user to perform basic photo editing, create interesting photo collages or design digital artifacts.
Sumopaint is another web-based image editing tool that provides the user with brushes, pencils, text, gradient and other formatting tools. It is one of the more popular image editing tools due to its ease of use and reliability. If you need a quick and easy photo-editor be sure and check out this free web based app.
I encourage all my students to install Infanview on their computers as it is an incredibly powerful image viewer. The free software goes well beyond just functioning as an image viewer. Students can use it for slideshows, basic editing, emailing, screen capturing and much, much more.
GIMP is one of the most popular free and open source image editing tools. It is an excellent resources for teaching both drawing and photo editing tools. Teachers and students can utilize it for projects where illustrating, photo retouching, batch photo conversion are necessary. Additionally, GIMP includes more advanced tools like layers, filters and effects.