I'll try to help with Gimp if I could. A couple of keyboard shortcuts to use by default are "O" (as in Oh) for the Color Picker tool. The other is the "N" key to use the Pencil tool. Plus some other stuff...
Problem with Pbrush is that I cannot find any transparent setting. If it's imported to GIMP then the white background is also considered as a color.
Also I'm using an older version of GIMP (2.8.16) and works well. You can get older GIMP versions at http://www.oldapps.com/gimp.php