I'm going to use jQuery. For several reasons. First it makes everything with the DOM much easier, not just for this but for everything else. Secondly it'll make the code necessary to do this specific task much shorter. Thirdly everyone else is using it and so should you if you want to be cool.
Of course this brings up the question, why are you doing this? Are the images randomly pulled or something? Are you using some sort of AJAX to get the image URI from the server because you can't insert it into the HTML server side for some reason? If not why not just insert the image in the div directly? If you're trying to make it fade in or something there's much better ways to go about this.