The movie database – TMDb – is currently working on a major image transfer, so all images have to be cached again on a new domain. This means that you may experience slow loading (or none at all) of covers and backdrops when updating your library.
It’ll slowly get back to normal once the image caches get filled up again. I’ll update this post when that happens.
More info can be found here.
Update: I believe this is more or less over now.