Sorry, in rereading my comment I see that it was ambiguous. I was referring to the use of IMG tags, which JimB addressed later (thanks!).
I should have used the [hijack] and [/hijack] tags. Mea culpa. (Aside – how about a [hijacker] smiley?? )
The smilies of course take up a small amount of overhead, but only once per session (unless the user doesn’t cache anything, which would be a Bad Thing).