How to create transparent building textures?

I’m trying to remove the surgical tools from Mercenary Camps, but when I make changes it adds a coloured background where the original was transparent (around the edge of the tablecloth and tools set on the table).

Does anyone know how to get rid of this? The .tga files seem to preserve the proper transparent alpha layer. Is it an issue with converting to .ddt?


For reference, the original is on the left, and what I’m trying to add is on the right (making the background black doesn’t help).