.blog-section a:nth-child(1) .blog-img {
    background-image: url(../img/sesja-rodzinna-i-noworodkowa/blog/Rectangle\ 9.jpg);
}

.blog-section a:nth-child(2) .blog-img {
    background-image: url(../img/sesja-rodzinna-i-noworodkowa/blog/Rectangle\ 11.jpg);
}

.blog-section a:nth-child(3) .blog-img {
    background-image: url(../img/sesja-rodzinna-i-noworodkowa/blog/Rectangle\ 12.jpg);
}