totally doable as a mod. i think i did it in the a mod called the 'naked shoutbox' over @ ubbdev (7.2.2)
if you want to do it to a 7.2 or 7.3, you'll simply need to modify the do_markup routine to handle it (ie: allow)..
7.3's version is much nicer/easier to modify, since Ian did his thaaang on it
keep in mind that for images, a user can link a huge one in and kill the look.. sooooo, you might want to add a check for image size and either prevent or just resize or even just make it a clickable link for big stuff
