Set background picture in blog section

Add to Design > Custom CSS

body[class*="type-blog"] {
    background-image: url(https://images.squarespace-cdn.com/content/v1/635d0687434dcc204d74ef76/1673190846931-KDBD0SYH138LND6FWMAJ/unsplash-image-ZWPerNlqUu0.jpg?format=original);
    background-repeat: no-repeat;
    background-position: center;
    background-color: transparent !important;
    background-size: cover;
}
body[class*="type-blog"] .section-background, body[class*="type-blog"] .section-border, body[class*="type-blog"] section {
    background-color: transparent !important;
}