Actually it was for setting a custom image in Steam's grid view for those of us launching it via Steam (by adding it as a non Steam game), otherwise you just get a big grey box with a tiny icon in the middle when using grid view,
I've never made an image for a Steam group before but if I can find out what dimensions it uses I can probably get something made fairly quickly, I'll edit this post with an attachment once it's done.
EDIT: Try out the attached image, its just one I found on Google and cropped it in such a way that Steam should resize it fine, if it doesn't, let me know and I'll upload a already cropped version.