In the DKP section of your guild site, you can add new dungeons which can each have a picture set with them.  The Global DKP dungeon however is not editable so you cannot set a picture that way, but you can still do this with a bit of CSS! =)

To do this load the theme for your site into our Theme Editor then click the Global Settings button and in the CSS text area add the following:

#dkp_dungeons_index .dkp_dungeon:last-child .avatar_image {
  background-image: url("http://s3.mmoguildsites.com/s3/dungeon_photos/2622/original.jpg");
}

This CSS simply says, on the DKP Dungeons Index page, using the last DKP Dungeon, set the image you specified; then you can just change this image to anything you like.

... and that's it! =)