Hayden Background Images

Site URL: https://hen-seadragon-whcf.squarespace.com/

I believe I would need custom CSS to add an image to a page background. Is this correct? If so, what is the code for adding an  png from my own files? The goal is to make it look like this graphical element (attached) is behind (and border) the page.

Password for the site is ex2Marketing

Thanks for any and all assistance!

Try adding to Design > Custom CSS

body#collection-61dc917fffc02b1e7c82e428 #page {
    background-image: url(https://content.invisioncic.com/p289038/monthly_2022_02/squares.thumb.png.817129e258affbf5788e6705f607e7f8.png);
    background-repeat: no-repeat;
    background-size: contain;
    background-position: bottom right;
}