Apparently this situation arises when and if the image cannot be reached, on the net.
In this particular case, imgur (that apparently hosts all images posted on SE) cannot be reached.
Sometimes, some domains are blocked, and most probably imgur is currently a blocked web domain.
I don't know how important this issue would be for SE. I've found out that it's not a bug that SE experiences.
However, this can be seen as a feature request to have redundancy of image sources. If one does not load, load the secondary one, or use a different server/domain for people with access issues.