Gimp is alright but not great when it comes to a substitute for Photoshop, imo. I've never heard of Affinity Designer, but by the interface it looks in depth. Inkscape is another that's pretty good for vector art.
I had gimp, and I deleted it out of a fit of rage because I hate the UI I'll look into the cracked version thing lol Free is better than $50
What kind of picture are you looking for? Just good wallpapers, or a totally custom drawing from scratch?