What is an "alpha layer", and how do I make one?

[Frossa]

Having some trouble getting the game to recognize my MapImage .dds file. After some googling and forum searching, I've come to the conclusion that what the image is lacking is an "alpha layer", as discussed in this ancient topic: http://forums.civfanatics.com/showthread.php?t=510042

Is it possible to make an alpha layer in Paint.net, or do I have to use Photoshop or GIMP for that?

 

[Wolfdog]

Yes you can in paint.net.
From memory you just need the download the plugin which a google search should find.