Maps2bgl problems

I have done my local airport, everything is fine. The problem is that I have a big square area that doesn't match the default tiles.To solve this, I try to fill in around the airport using pure white to cover up everything that I don't want to show up so that it will blend in better with the default tiles. I then save the white area to an alpha channel within the .tga before I compile everything.Once back in FS, the airport is there in the right shape that I cut out, but now filled with water in some tiles. Everything outside of it is white, all the way to the square borders. It is like the white is not being recognized as transparent, not letting the default tiles show through.If I invert the mask, then the outside becomes water and the airport shows up okay in some tiles, but white in others.Either way, I've not been able to get any of the default terrain to show through. Has anybody figured out a way to do this yet? Isn't there a way to mask over land areas that we want to show through the new photo scenery?CH