Making a new Main Menu image

[Ambreville]

I tried replacing the background image in the Beyond_The_Sword_Main_Menu.dds file (in the mod's Assets\Art\Interface\Main Menu folder). I used an image that is the exact size (1024x1024 pixels) as the original, refreshed the screen after edit, and saved the file as DDS without the mini maps option. I use the DXTbmp application for that. The game bombs when I reload it with the modified image. Any idea what else I need to do to get that new image to work?

The same happens with the dds file located in the Assets\Art\Interface\Screens\Loading folder.