First of all, welcome! Secondly, why do we do this? Probably because most of us have quite a rich imagination, and this imagination manifests itself in my slightly crazy head thinking up exotic and strange fantasy-lands. I guess it's quite easy to understand that some people who have this affliction want to map what their heads think up, before they get lost in their minds
But in the end I think I can speak for everyone here that we make them for fun, and usually from scratch.
For how we make maps, that's what this entire forum is about! I'm no expert on GIMP... or rather, I know zip about it. But a lot of people here do! That's what the forum is for, for answering that exact question. Which is slightly more complicated, since that fickle little sucker called taste comes peeking around the corner. Go look at what people here create, and figure out what you like, and try to look for tutorials on how to do that in the appropriate sections. This forum really is a treasure chest for people who want to artistically map things... I hope you have fun and that you manage to answer that last question by yourself.