What's qualified as disgusting? I've seen many nasty pics not tagged with disgusting tag
Disgusting is anything you don't like. It's is subjective, but if you limit the tag to abuse-only works, other people will agree with your subjective view at least 9 out of 10 times. If you see posts without a tag that might require one, just add it; if you are not sure, post a comment to the picture or open a thread on the forum: someone with more experience will probably help you.