Are you talking about "logo.jpg" or "bodyBg.jpg"?
to call a PHP script ... otherwise, it would probably be better to use pure PHP.
I mention PHP for other reasons ... future CMS (content management) and
swapping? And how should it swap ... randomly, by day, by week? What it the
criteria for swapping? And do you have other images in the same directory? Give
us the filenames for some of the othe images it will be swapping.