A couple ways you could do this. One is you could use an HTML background that has a timed re-direct to go from a blank page to a page with an image after 30 seconds (or whatever).

Another way would be to have two image items back-to-back. The first would play your sound file as a background sound and have a duration parameter to advance after 30 seconds to the second image item. The advantage here is that background sounds carry over to subsequent items until they end or until a new background sound is encountered. This is probably the way I would go because it would be super fast and easy to implement.