[userbinator]

Apparently there are people who think it's "aesthetically unpleasing", which doesn't make much sense to me either: http://forums.mozillazine.org/viewtopic.php?f=8&t=63343&star...

There's also this related bug which contains over a decade of discussion:

https://bugzilla.mozilla.org/show_bug.cgi?id=41924

I agree that it makes more sense to give the indication "here is an image but it couldn't be displayed" than to have them disappear completely.