I was just looking for this myself, I wish it was at least an option so that images in posts had a unique wrapper.
I have a use case where one of the themes available is for obfuscating what you are viewing (at work, in public, etc. Think GhostZilla) that uses the lightbox wrapper to CSS target post images and hide them until you mouse over.
a.lightbox img {visibility:hidden};
a.lightbox {background-color:#eeeeee;}
a.lightbox:hover img {visibility:visible;}
a.lightbox:hover {background-color:transparent;}
a #site-logo {visibility:hidden};
a:hover #site-logo {visibility:visible};
header div.title a {background-color:#eeeeee;}
header div.title a:hover {background-color:transparent;}
a.main-avatar img.avatar {visibility:hidden !important;}
a.main-avatar {background-color:#eeeeee !important;}
a.main-avatar:hover img.avatar {visibility:visible !important;}
a.main-avatar:hover {background-color:transparent !important;}
.posters a img.avatar {visibility:hidden !important;}
.posters a {background-color:#eeeeee !important;}
.posters a:hover img.avatar {visibility:visible !important;}
.posters a:hover {background-color:transparent !important;}
.topic-poster a img.avatar {visibility:hidden !important;}
.topic-poster a {background-color:#eeeeee !important;}
.topic-poster a:hover img.avatar {visibility:visible !important;}
.topic-poster a:hover {background-color:transparent !important;}