Advance Wars: Dual Strike

Taken from the cheats section:

Unlock final wallpaper in design room:
Obtain all 300 medals

If you are having trouble with the Brutal combat medals, my best advice is this:
Andy is easily the best CO, so go with him. Go with with either all tanks or all tanks plus 5 mechs. The easiest part to die on of all missions is the beginning where they all rush you. You can use mechs to dish out some damage before they die then kill everything with tanks, or simply start with tanks. After this, always advance slowly and run away to heal on properties always. Always capture bases with tanks to get more of them. Otherwise, the rest is general skill I suppose. It goes without saying that you should also be using the touch pad to always shoot so you can run away while shooting, etc.

To answer the main question, the last wallpaper is the same as the ending screen after beating the hard campaign. The picture is of all female COs in their alternate costumes.