I've been using GIMP to make every one of my thumbnails for the video uploads I make. The issue with GIMP is that it does some things a bit obtusely, and kind of requires you to build a database of art files. Photoshop is better for (surprise!) editing photos, where as GIMP is better at pure "Art" stuff.

GIMP also is not really geared for Tablets, which is also why most people are down on it.