Looking good B.A., and I am looking forward to see something from the game you will be making using it.
I have done a lot of editors with blitz, but yours look pretty darn good so far. Something you might consider, depending on what type of game you will be using it for, is having layers for doing tilemaps that are layered with a background, movement and foreground layer, so that you could have tiles that the player moves infront of, tiles that he moves on, and tiles that moves infront of the player. That could be useful for others, if you plan on having other use the editor as well. Also an indicator of exactly where the tile will be drawn would be nice, as I mentioned on another forum. Feel free to send me an email if I can be of any help should you get stuck on something.
@Amon, not sure if it will help, but here is a map editor I made for someone who was doing a breakout type game, but the idea is the same for any tile type editor. It have tile picking and placement. Everything is coded using functions to make it easy to see what does what, but there are no comments. Feel free to ask if there is something you don't understand.
http://zac-interactive.dk/temp/PLOID.zip