I'd also suggest GIMP, but for simplicity I'd suggest downloading and using an older version like 2.6. Unfortunately, like many other programs, as the versions roll along, GIMP seems to be needlessly adding complexity that steepens the learning curve. For example, in version 2.8 they introduced a "feature" where you can only "save" files in GIMPs native format, but have to "export" them if you want the file to be saved in a common format like BMP or JPG.