Campaign Editor Question

Hey there, i'm rather new to the Campaign editor, and the making of campaigns in general, so i have a question for you all.

Under the tab Loading Screen, you are able to set the background screen as you all know. Of course there's all the nifty 3D backgrounds, but there's also some 2D backgrounds, all from the single player part of the game.

Now i'm wondering; Which format do you have to use if you want a custom Background in your campaign ? I've tried both .tga and .blp, but as i suspected, none of them works. So i would like to know the format used for the backgrounds, in case anyone missed that part.

Thank you in advance