does prety much what it says on the tin, it loads jpeg files... but only in modern skins, since its a wac and is only in use when gen_ff is working, which only happens when your using a modern skin
and it works just like carlosp said
<bitmap id="jpg.file" file="player/main.jpg" gammagroup="somegammagroup" />
works exactly the same as png, except its a jpg.
and before anyone asks, yes, you can use jpg and png in the same skin