@Cordovan - It just happened again.
I was paying more attention this time.
I had been browsing the forums for ~30 minutes without issue. Then I noticed I was starting to briefly see the three giant letters for a split second before the avatar icon/image loaded.
When I loaded the page in the screenshot, I guess it got stuck or timed out and didn't load all the images. After I Ctrl+F5'd a few times, the images loaded normally. I'm guessing that's what it defaults to if it can't load the avatar image files.
So it seems as if this happens when the webserver might be lagging a little bit? All other images on forums.ddo.com loaded normally when this happened. It was just the avatar images that didn't.
I use Firefox if that helps. I've had no problems on any other websites or forums. I've had no recent issues with my inet connection (I get alerts when it has high latency and/or packet loss). My PC hardware is no slouch.