custom image menu to show subpages in drop down

I have made a custom navigation menu which I want to use rollover images on. I don’t want to use the general wordpress menu snippet that displays all pages and subpages. Instead, I want each image/button to have code that says “display subpages of parent page x”. That way I can specify what the parent page is for each button and it will ONLY show the subpages in a drop down.

I don’t need the css, just the php. Any ideas? Thanks!