Wall Placement? (Very, VERY new to this...)

[Deimos]

So, I've used a few free web-based services to build maps in the past, and recently I moved to MapTool. I have downloaded a ton of images available under the Creative Commons, and have been doing some fiddling around trying to get oriented with the software.

When I first tried to place a wall over a floor tile I had laid down, I was surprised that it was in the middle of the tile. I thought I'd got a bad batch of images, and so I went to a different one. Sure enough, middle of the tile. Same with all the rest. I tried with a different floor tile, too. Same thing.

Is there a reason for this? I want my walls to be on the edges of my floor tiles, not in the middle. What is it that I am expected to do that I clearly am not doing?

Thank you!

edit: Pic related

D&D-mapping-wall in middle.jpg