I can't tell you if they'd be okay with it or not, but it would be trivially easy for them to stop it if they weren't. So, I wouldn't worry about it too much unless you are abusing it. Also, the practice is generally frowned upon when one large website officially hotlinks to another one, i.e., steals its bandwidth to serve lots of images, applets, etc. Individuals (like us) linking to single images on websites from a chat forum is an entirely different thing.

On the gear sites I visit, e.g., bladeforums, candlepowerforums, etc., people hotlink all the time. I've never heard of any vendor blocking it, most likely because it brings in business in the long run.