I have to design a website for my friend and I have no clue how to change the menu image if the page scrolls down, for example

If the person clicks on a submenu it will take them to a specific anchor within a page, I have setup a custom jquery scroll bar so all the contents are in the Div that is scrollable.

The website is: http://www.cobrie.com/adstest/

If you can see that if you click on the submenu which is 2nd hand and repair and rebuild the content will scroll to specific area, but the clients don’t know there location as once the sub menu is clicked the original submenu image appears.
So I want them to be able to see if they selected the 2nd tab, the button will stay highlighted and if they scroll down the page when they reach another heading with an anchor the corresponding submenu tab becomes highlighted.