All you need to do is make the images that you want in your tileset (or just edit the existing RTP), insert them in a new image that is at least 256 x 576 (the size really doesn't matter as long as it is compatible with image sizes). Save it as .PNG and import it to your database.
Open up your database, go to tilesets...create a new tileset and set the image for the one you just created...set the passability and all that other good stuff.
Cheers.