I think it helped to show the background images with 50% transpa... sorry luminosity. I don't really mind if the images are clipped by the window, but I can understand that some people do. Perhaps it would help to put the center of the image in the center of the window?
Another thing I thought about regarding placing the small build icon was to put it in the upper right corner, since the tabs leave that empty space anyway. However, unless it is made distinct enough people might mistake it for another tab, or make the window seem to cluttered.
For the official tags, see the right side on https://bugs.launchpad.net/widelands/ . When I get round to it, I plan to write some documentation regarding bugs and the bugtracker.