Not OP, but here's my experience: It's very rare on my end and happens while browsing for posts that link to a website. I wouldn't really classifiy this as high priority, since I suspect that it's the fault of the website, and the wording of the error message in Eternity simply leads one to believe that it's exclusively Eternity's fault for not loading the image. Here's an example in Eternity:
I found out a goofy ahh solution for it: click on the post to go into the comments and then click on the separator bar thingy under the image that doesnt load and that opens the image in the image viewer and most of the times that loads it for some reason.