If anyone is interested, I have created a small RGBA PNG that is identical to a checked out tile, except that there is a transparent area at the centre, where we are supposed to draw. This area is not completely transparent: some pixels at the edges are semitransparent, following the antialiasing in the original tile.

The image is: http://img228.imageshack.us/img228/404/bordernm4.png
You can use it as an upper layer/level in your drawing program to mask the parts that shouldn't be visible.

The complete available area (including both completely transparent and semitransparent pixels) is 240x178. Of course nothing forbids us from breaking out of this "jail".