There are pros and cons to a generic background to pass the blocker status. The benefit of course is that it fulfills the need to be a different background and thus detectable as not the latest/stable etc. The cons is that quality of that generic background image may be less than it currently is, and that could change perception of the quality of the Alpha itself to anyone outside the core group who experiments with it.
So, if we could come up with a high-quality, generic background image that is the image we always use for prerelease etc, then sounds like a good compromise. There wouldn't be a holdup for new artwork, yet it would look different and thus noticable as not stable.