Just did a little test run using GIMP to create a very simple dungeon map. This took me less than 15 minutes to make. With a bit more time, and some more practice with GIMP making maps, I should be able to knock out some more complex structures, and add more details to the dungeon. The map I made yesterday for Under the Hillfort Ruins is more complex. It has doors, statues, stairs, and other features marked on the map, along with room numbers.
*Note, I doubt anyone would want to publish an adventure with this simple little map. Just planning ahead.