A very good start. Did you make this map in Dundjinni, then import it over for post processing in the GIMP? Or did you make the map entirely in the GIMP?
A few suggestions:
1) Is there an escape in the top of the room? Or is there a light source from the ceiling? You picture suggests that. One thing you could do is lower the opacity of your shadows a bit, and drive up the spread. If it was me, I would take out the drop shadow all together, as it does not sell the illusion that the walls are in a dungeon. If the GIMP has the function, try applying a inner glow instead. This will try into my second suggestion below:
2) I would not cover up the walls with the texture of the stone. Have them go right up to the walls. If there is no drop shadow on the outside of the walls, it will sell the illusion that this is a dungeon better for you.
I will try to do something to demonstrate what I mean for you after school... as I need to go now.
A great first map. Keep it up.