I'm not a techie, but as I understand it, it could be either. Maybe the server is briefly failing to deliver the image file, or maybe a bad file's got into your cache. (Refresh on its own won't necessarily fix that. I think sometimes you might need to do an unconditional refresh - CTRL + F5 - or to manually clear the browser cache, exit the browser, and then restart it.)

To be honest, I think just about every website of this kind occasionally fails to deliver an image file, so if what you saw is consistent with my surmise, personally I wouldn't even think twice about it.