The GNU Image Manipulation Program (GIMP) is a great bitmap graphics application, one of the most widely used FOSS for many years, kind of solves the problem in this space. Gimp is very powerful, but the interface has been optimized for ease of use so many fundamental actions can be done quickly with keystrokes. All FOSS, licensed under the GPL.
GIMPshop is a modification of GNU Image Manipulation Program (GIMP) intended to replicate the feel of Adobe Photoshop. Also all FOSS, licensed under the GPL.