Well not having anything else would do the trick.

Gimp is totally overkill, and I actually do agree it would be a bad choice for you. For such dungeon maps, I think Inkscape may be a good choice. Or try paint.net, it's not perfect, but nicer than ms paint and it's free, too. There are probably other alternatives. But in the end, whatever works for you is the right tool, really. Thanks for the explanation tho.