Does Game Development have any tutorials for this kind of stuff? If it doesn't, you should look into it. I think it would be a great addition to your forum.
Open up that thing you call GIMP, put on a 32px x 32px grid, stretch that tile by 200%, align it with the grid, and it should be all set. although, you may want to do that on the tileset that you're using, instead of a seperate one.