It has become clear that there is another issue in play here; not just missing images.

So, for missing images, we are logging "Cached Image Missing" for those from now on, and will be using "Image Retrieval Error" to indicate a failed connection or lack of response from our storage provider.

[GEEK-SPEAK]
After a couple of weeks of in-depth troubleshooting on our servers and working with our storage provider's engineers as well, we believe that we have finally gotten to the bottom of the "Image Retrieval Error" issue (which has been increasing in frequency).

It took a lot of debugging and error reporting, because in the overall scheme of things, this issue was only occurring about 1:1,000,000 requests, which made it extremely unlikely to manually reproduce the error condition. We also had no way to "catch" this error LIVE, while still gracefully failing on the client side, since every means to gather error details resulted in NO data at all. Short of throwing errors in the LIVE service, I finally stumbled across the thought that it might be rate-limit related and was able to reproduce this error condition manually. It was then that I learned that these headaches have been a result of intermittent rate-limit DNS failures when using the more popular Public DNS services.

Now, we are working with our Datacenters to implement a stable DNS solution that should finally prevent these errors in the future. We expect a resolution within 24-72 hours (since it is a weekend now).