There is currently not a tileset system. There used to be, thanks to the awesome coder Gremour, but he has retired. We would love for there to be one again, but none of us have the time on top of all the other things that need to be done. We all agree we should have support for tilesets available, though, and it isn’t actually as difficult as one might suspect because a good portion of the work has been done for us. There is still work to do, however.
And you can help.
If you want to see tileset support, start here: http://whalesdev.com/forums/index.php?topic=1170.0
Anyone who can bring Gremours support up to date or come up with their own implementation, it WILL go in the game. So get 'er done! Linked in his thread are also the two most advanced tilesets available, one of which we will likely end up packaging with the “default graphics pack” version of the game on the front page. But none of that can be done until the code is made compatible with DDA.
So come on and help us out, and get tiles working with the game! Post your results in the Mod forum and if it works well, we’ll roll it into mainline.
To discuss this, post here: http://www.cataclysm.glyphgryph.com/smf/index.php?topic=202.0