I'm having problems trying to create irregular mosaics. I've created a mask image like the one in the instruction pdf (the spiral). I'm using Photoshop, white background, black brush, tried different point sizes. The problem is that in the final mosaic, the spaces inbetween the lines are black (should be images), and the lines themselves get filled in.

It's correct. The black color (brush) is replaced with images. The white color (background) is replaced with the background color that you set in More Options of AndreaMosaic.
Make a maskfile using for example blue as background and then white or black for the brush.
Check the latest user manual (originally the algorithm was different):
http://www.andreaplanet.com/andreamosai ... Manual.pdf

P.S.: I'm very sorry for the delay, I was away in these days.

OK, it's working now, thanks for the help.