I just followed the 1st post in this thread that says:
The theme is action! We want an exciting battle scene, set up in either OFP or ArmA, which shows what the game in question can look like at its furious best. Images can be up to 1024 x 768 pixels in size, but be aware the image will need to be cropped to 950 x 150 pixels to fit in the banner space.
It doesn´t say that the banner has to be done, it asks for images to make a banner of.