First, thank you very much for excellent instructions and motivation (oh and your art has been very helpful as well in my MapTool-centered campaign.) I even used your sample dungeon in a mini-adventure in 4E with some folks and we had a blast. (If you want the .campgn file, I can send it to you.)

I have been using GIMP 2.6.6 to try to recreate the steps you've outlined, and I must be dense--I select "snap to grid" (after setting the grid to 50px x 50px and showing the grid) and the paint brush fails to snap to the grid. I've checked the GIMP Bugzilla and there is no mention of this being a bug. Am I missing something obvious? I suppose I can just do caverns until I figger this out. ;^o