[Tyhm]

I've made it as easy as I can - if you download the layered image, you can turn off the Background Layer to isolate the pictures that are actual players, rearrange them into a conga line or whatever, paste in a new background layer (a screenshot of the castle or the editor or whatever), then just crop it to Banner Size and call it a day.

In fact, here's the Final(ish) Template, with as many people as I could cram in, with the background layer turned off and a layer to letterbox just the Standard Banner Size, in the fairly-universal Photoshop format. (Pretty much any graphic editor stronger than Paint should be able to handle it)