jQuery shortcode to shorten width of a preceeding DIV.

I use shortcodes in wordpress posts like [chat name="Brittany"]Hello there.[/chat] to create some fictional dialog between cartoon characters, like on [login to view URL] . I want a new PHP shortcode created to go around images that I will put into my blog posts, and I want the PHP shortcode to use jQuery to find the preceeding chatbox's DIV, and make it smaller, so that the image can shift up and sit beside the preceeding DIV, not below it or behind it. I've been having trouble getting $(whatever).prev() to work, so I want to turn this little job over to someone with more time and experience with jQuery and PHP.