nsImageLoadingContent::Notify is where all image notifications for things like <img> elements go through. The vast majority being implemented in C++. Any image observers implemented in JS must go through ScriptedNotificationObserver.
We only use ScriptedNotificationObserver in tests. The addon repository only has six hits, and four of them seem to be different versions of the same addon. And they don't seem to be among the more popular addons.
The original location of the assert only caught some images anyway. Things like CSS background images don't go through nsImageLoadingContent. ProgressTracker is the origin of all image notifications.
Boris Zbarsky