How hard would it be to change this so that the designer can specify the size? Do they *have* to match the sizes you've used? The problem is that you've chopped up the bitmaps in a different way to the original DM, and everything is ever so slightly out, so it's impossible to get the original DM tiles to work if you use them out of the box...
Additionally, if you used DM methods of drawing and allow the designer to override the positions and offsets, it would make developing seamless texture sets for RTC easier than at present and would allow the 'bandages' that Cowsmanaut discussed without requiring ANY EXTRA images (through clever use of transparency and knowing what the lower images will do when upper images are overlayed). Stuff like using the bushes images from DM2 will be very hard to do for RTC at present...
Here's what I mean (The green bit would actually be pink to work correctly) 
It's also worth noting that the RTCs layering of walls wouldn't be affected by this, assuming you always draw from the back to the front and from the sides to the centre.
I've been messing with this a fair bit, and am curious about why you used your method.
It seems odd that you would add all this flexibility for change, yet the walls are still hardwired into the engine...