Relic Castle

This is a pretty basic question, but how do you import custom tile sets?

It seems I can't access the custom tile sets I import into the game-- in "map properties" it still just gives me the same set of selections I had before importing the tile sets, and I don't know how to fix it.

I also asked this question on the RPG Maker forum.

Edit:
Now I know how to import tiles, but it's still annoying because I can't mix the content in tile sets, like what would I do if I wanted grass in a cave (besides photoshop editing), for example?