It's the effect on our pagination/request times due to the call out to other servers. Those of you using certain browsers can, for instance, see the hang time while a request to another server for a graphic is complied with. The most noticeable being those of the adverts appearing at the bottom of the page. Slows the server, the page completion etc. Leg bone connected to the knee bone kind of thing. I'm afraid we're stuck with the ads to pay for the bandwidth bill and therefore making callouts to other sites for pic's, graphics and emoticons a cause for some thought helps us keep the server running relatively smoothly for longer.
Our server is not even 10 months old and is already showing strain for a small but significant part of the day now. We have a range of pretty but non esssential options (aka 'eye candy') we can begin switching off to help extend working life - the last one we ever use is knocking out the img command altogether.
Please note - we don't make adding pictures impossible - just laborious enough to make the average PPRuNer think carefully as to the value/impact of the image.
Regards
Rob