To change background image of One Page in York Template Squarespace, you can edit page > Add a Block > Choose Code > Paste this code. Replace Pixabay with your image url.
<style>
.overflow-wrapper {
background-image: url(https://cdn.pixabay.com/photo/2023/09/26/17/32/woman-8277925_1280.jpg);
background-size: cover;
background-repeat: no-repeat;
background-position: center center;
}
</style>