Photoshop is market leader in image editing but there are free alternatives:
GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
Not as complex as GIMP but not everybody's needs are complex. It is simple and free.
3. Pixlr Editor
Pixlr Editor is part of a suite of Pixlr apps, including Pixelr Express and Pixlr-O-Matic.
Pixlr Editor is a webapp that offers a good dose of editing features. It packs adjustments, layers, filters and effects, basic features like rotation, resizing, cropping and editing, it's free and runs in your browser.
Picasa is an image organizer and image viewer for organizing and editing digital photos, plus an integrated photo-sharing website. It is cross platform and free.
Sumo Paint is a free online image editor, without need to install anything to your device. It's versatile and works in a browser. You can open and save images in hard drive or cloud.