Scrolling and fixed positioning

Hi all,

Really hoping someone can help me here…!!

I have a landing page with a full screen fixed position image. I want the image to remain fixed until the page has been scrolled down to 300vh (from original 0 position) then I want the image to scroll up with the rest of the page…

How can I achieve this please…?

Here is my public share link: LINK
(how to access public share link)