Steam. Yes, I deleted and redownloaded it. Ruined 2-3 Invasion and 3-5 Bounty matches for me. I posted other screenshots in Cool screenshot topic.
It first started happening after Pet Battles. Seems that I opened Troops menu and clicked to Pets tab before all the images were downloaded (these images are being downloaded every time I restart the game).
After that it started happening outside of Pet Events.