I'm not an artist. I'm certainly no where near many of the members' level of talent on here. I just use GIMP, but not very well. XD
Here's the base of my map (3,000 x 3,000): http://img706.imageshack.us/img706/6050/shadowyrm6.jpg
First, I painted black walls on a white canvas. Then -- obviously to those who know of it -- I used RobA's Subterrainian Map Prettier Script. After removing the square grid, I sandwiched a hex grid between the wall and floor layers (we play GURPS Fourth Edition, which uses hexes).
Then, I freehand drew the little elevation stairs all around and used the drop shadow filter on them.
That's it so far.
Eventually, I'll remove the wall texture and use filters to create my own with the help of the tutorial, "How to Blend Textures for Use in Battlemapping with GIMP."
Next, I'm going to add stalagmites everywhere. Then, I plan to blend in some dirt texture (created with filters) on the floor. Not sure exactly how I should do that, I just know one single texture for the whole floor is boring.
I'd love to add mist, but I've no idea how to even start with that or how well even an expert could pull it off.
Dramatic lightning would be great as well, but again, I'm a total newbie with little experience and no talent.
One thing I really want to accomplish is add water in the center room on the bottom "row" of rooms using only filters, no textures. Though I worked through a good tutorial once, I can't seem to get results that please me. If anyone can help, I'd appreciate it! I want to add a pool of water to the north and south of the "bridge" in that room.
I might break down and add furniture and stuff, but I typically use minis to represent things like that. Also, I have no resources at all, rugs, tables, urns, boxes, books, piles of coins -- none of that stuff. I'd have to spend hours searching for it all.
Well, thanks so much for talking the time to read my post and look at my map! Advice is greatly appreciated, but remember, I barely know how to use GIMP at all!
EDIT: Latest version: http://img854.imageshack.us/img854/7528/shadowyrm8.jpg