Set height of background to 100%

Hello,

I wonder if anyone can help me- I am trying to create a sketchbook look on my website and have uplaoded a background which acts as a border to the page, to look like the outline of a sketchbook.

I did have it set to Cover, but then it looked different on every page so I switched it to Contain, but now the bottom of the image is visible before the end of some of my posts. I would like it so the width is always the same but the height is automatically adjusted to each page- I don't mind if the height is cut off on some posts.

I have tried uploading an extra long background image but squarespace is having processing errors with it. Any help would be much appreciated.

Thanks

https://greyhound-keyboard-sa3k.squarespace.com/