Sure, but the point is that their business is not viable then. If it's illegal to serve childporn and imgur can't profitably detect if it's serving childporn or not, then imgur is doomed. I think no one would dispute that?
If the answer is 100%, then yes, all user-generated content on the internet will have to be banned unless subject to manual human review before allowing the post to be seen, and even that is probably not perfect.