Semi random image from my comments folder
(seems suitable for this thread)
"Hotlinking", directly linking to images hosted on other sites that are not expressly for that purpose (such as imageshack etc).
This is because:
1: It's considered rude.
2: Some site owners get quite upset about it.
2A: Some sites have measures in place that replace the hotlinked image with something very unpleasant if the person viewing it doesn't have a suitable cookie showing they have been to the site the image is from.
It's primarily 2 and 2A that have us implimenting the no hotlinking rule, in the former case to try to avoid problems for other sites, and in the latter as we will suspend people if the image displayed is unsuitable for the forums, even if it wasn't their intent (users are responsible for making sure stuff they post on here falls within the rules).