I like the initial Idea, but I have a few suggestions:

1: As Scakk suggested, a way to remove the background
2: A way to specify the font to use and size
3: A way to specify the color used
4: A way to specify whether the picture be centered or photoed(or titled, etc)

A few things to improve your current code:

;Saves you the trouble of having the if statement
on Me:*:JOIN:#: backnick #

;If there's no pipes/if/while/etc statements, I like everything on one line.
on *:OPEN:?:*: backnick $nick

;Just, imo, cleaning up the code:
on *:NICK: if ($window($newnick)) backnick $newnick

;Instead of /bquery, I just replace mIRC's built in /query
alias query !query $1- | backnick $$1

I give it a 7/10 but get some more user options and I'll up it.