As an event called Texture in the Briefing Room opening settings. It must be a square .jpg
(256x256, 512x512, 1024x1024)
In the editor, right click on a side's mission name and choose Add Event->Texture, then choose on your pc where to find it. Real easy. Also, you must set a curtain event (or the curtain won't open) for the same time as the texture event, so they happen at the same time.