Don't use Paint, as it doesn't allow transparency.
Go for Paint.net (which has a similar interface) or if you're after something more complicated, GIMP. With Paint.net, you can download a plugin that will give you a grid layout, to make it easy to place tiles. GIMP comes with a grid layout out of the box.
I think I'll just go ahead and merge this with the topic for the appropriate ReStaff release