The GNU Image Manipulation Program, or GIMP, is a popular free raster image editor. Inkscape is a free option for vector graphics. Default image editors, such as Paint for Microsoft Windows, are good for simple edits. Artists may also be interested in the free Krita and PyxelEdit image editors.
GIMP is a useful free alternative to Adobe Photoshop, according to DigitalTrends.com. It is available for PC, Mac and Linux, and it provides various professional-level editing tools. Krita also includes numerous tools, making it suitable for editing images, but it is more geared toward digital painting. PyxelEdit is useful for images with limited palettes, such as pixel art. DigitalTrends lists other free image manipulation programs, including free online image editing options, such as Pixlr.com.