I'm not aware of any other apps other than GIMP. I'm not sure the learning curve for GIMP wouldn't be any more or less complicated than other comparable applications for someone who's starting out and doesn't have any experience.
The are services online, like Aviary
, but one would still have to learn those tools and much of what one would find in one application, one could find in another which might be cheaper [or free].
There's going to be a learning curve regardless. Might as well at least learn it in GIMP and shell out $ later if they want/need more.