I think the name concerns are legitimate just on the basis that it sounds awkward, regardless of whether it's offensive or not. An image editor named Gnome or Goblin, or even Midget (widely regarded to also be offensive), would be fine, because those make me think of cute little people. A 'gimp' is a weird guy in a leather suit or a guy with a messed up leg, and I don't really want to think about any of that when I open my image editor.