For just starting out I think you are doing fantastic!!
I would add some highlights to the mane and tail to give them more of a "moving and flowing" look.
Also the stirrup leathers should be at a slightly different angle.REC has a wonderful tutorial that sort of shows what angle they should be at.
This tutorial is super in general! Im sure most concepts can bee adapted to work with GIMP.
One of my other favorite things to do that I think greatly increases the realism is replacing the Sims sand with a real photo of arena sand
But you're doing awesome!