Zeev, signature pics don't take any space at all on iHub, because iHub does not offer storage -- except for the text of posts and such.
All graphics, whether charts or pictures, have to be stored somewhere other than iHub, (since iHub does not offer storage). I keep mine on a friend's free webspace that she gets with her internet service.
As far as bandwidth usage, they are pretty small graphics, so they don't take much. Plus, a browser will only pull a graphic up fresh the first time it "sees" it. After that, it will look for it on its own hard drive in the browser cache.