GNU Image Manipulation Program or GIMP is a highly regarded FREE image editor. Currently in version 2.6.6, GIMP accepts the vast majority of graphics formats. It has robust layer and masking tools. Function is extendable by use of freely available plugins.  GIMP shares or emulates many of the functions of high-end applications.

The GIMP distribution website can be accessed by clicking the image.  Many other support and plugin resources are available via the same link.