An awful lot of challenges I look at have the little preview “snapshot” with a background but no ships:
Not the most informative thing. It’s not across the board by any means, and there are a few that go even further and give just a black square.
Thoughts on what this might be?
I’m sure the black squares are challenges made before Cliff added the preview feature to challenges so they have no pic to display.
Not sure about the invisible fleet with the background though. Maybe they’re using some form of (raspberry) jamming technology. 
Bit of an update and threadomancy as well.
I’m still seeing this, on every retaliation I get. These are on a custom map, so maybe that is part of what’s causing it, I don’t know. The fleet snapshots seem to be mostly working OK on challenges I pull off the challenge list. This is really annoying though. Any more thoughts on why it might be happening?
Is it possible that you don’t have the race the challenge was made with, but that the background is a vanilla one?
I’m seeing this too. You might be correct in looking at retaliations.
Looking at it now, recent personal challenges not sent from myself are blank (with background). Follick’s (4583118) public challenge appears to have been created at least initially from a retaliation (with a name change to ‘all’) and is also blank.
Most of the retaliations that are having this problem are vanilla races; plus I have all the expansion packs. So probably not the problem, but a good thought.
I’m glad I’m not the only one seeing this; makes me feel a bit less crazy.
actually i had a problem once with the preview; the enemy race was the federation, it was a blank background. I set up my fleet, even though when I went to the battle screen there didn’t appear to be any enemy ships. I could fight though, so I pressed the “Fight” button…
…it froze upon loading and crashed.