Images over lapping :(

Hi i’m building a personal portfolio located here. I need you’re help! https://jamienicholl.co.uk

I would like to make the background picture top and bottom half join up on scroll to the bottom of the page but i’ve run into a few problems with the top half over lapping the bottom on different screen sizes and browsers any suggestions are welcomed thanks!!

p.s Top half should remain on screen at all times on scroll.