If it shows up white the file is not found,
I added my scenario TGAs into the texture folder of my mod (since you can only play the maps with the mod)
as long as the mod is not running the pics show up white, but if I started my mod the picture is shown.
so maybe you have not the correct name of your picture in your map file.
be shure that the spelling is correct and remove ".tga" and any path if you have one in you reference.
i havent tried yet adding the scenario pics into the normal texturefolder, but i think this should also work.