Proper way to do validation before adding to a queue

I need to check some things about a node before I add it to the queue. (specifically I need to make sure that a CCK image field is populated.) I have got it working the way I want by directly changing the code in the nodequeue_admin_add_node function in nodequeue.module, but I suspect this not the proper (or at least preferred) way to do it. Is my suspicion right? If so, is there a better way?