This is ... considered to be bandwidth theft in situations where you would put this image on your own website without the permission of the owner.
Yeah, good point. I got scolded in a forum for doing that somewhere along the road.
These days I only do it if I'm promoting a product that the source site sells. Even then, if its something I post a lot of pics of (like Dougs Mk3 fixed blade knife), then I'll just download the pic and upload it to my own ImageShack location - to avoid pulling bandwidth from someone.