Re: Forget the geeky stuff, sort out the user experience.
The unabbreviated name does explain what the IMP does, it’s just that its full acronym (including GNU) is one that can’t really be referred to in polite company (or, perhaps worse, in a business context: do they want many people to use their otherwise excellent software or not?).
I think this is one case where losing the GNU from the name really would be a definite step forward (much as I love the tradition of punnery in many open source software names (such as mail clients traditionally being named after dead trees (but of course))).
Besides, as anyone who has ever visited Discworld is aware, it’s well known that it’s imps who do the hard work behind the scenes of image manipulation, not rubber clad dungeon dwellers…