[Redwizard]

Zero.. Not very often is a jpeg used with transparency's. PNGs or Gifs or such are the usual graphics for this. When saving a graphic as a jpeg, try saving it using photoshop, set the quality to 12 (as i do) and there is no quality loss at all, only when you try to recolour large areas using a fill can or some other similar colour selector.
For the purpose of presenting a graphic in a forum, transperency is not essential, and the image that is viewable to the user is not significant enough to say "ohmygod its rubbish look at the quality". Upon maximum scrutinisation, where i have zoomed in to the max on this graphic in its jpeg format, i can only just see SLIGHTLY lighter pixels of colours.
So the quality may be reduced to a small extent, but by no means is it drastic enough to have a serious impact, or even a small impact, on a graphic.