There's just a common sense policy, really. As long as the picture isn't absolutely gigantic it isn't really an issue.
And of course, host them somewhere you have control of ideally. While nobody's going to slap you for hotlinking to someone else's server, if they get all mad with you for doing so and replace the picture with something disgusting it'll be you who ends up seen at fault.
These kind of apply to pretty much any forum really.