Dweep: If you follow this link you can download an "empty" file with your drawing set as background image.
Emmanuel: There seems to be a bug in the latest beta. When drawing a wall it isn't possible to edit it. I found out when importing Dweep's PNG as a background image. I then checked with an older design of my own. I even restarted the computer but couldn't edit the walls anymore.
I then tried to revert to an older beta, but without succes. I hope you have a solution
Hans