.blog-section a:nth-child(1) .blog-img {
    background-image: url(../img/sesja-ciazowa/plenerowa.jpg);
}

.blog-section a:nth-child(2) .blog-img {
    background-image: url(../img/sesja-ciazowa/minimalistyczna.jpg);
}

.blog-section a:nth-child(3) .blog-img {
    background-image: url(../img/sesja-ciazowa/rodzinna.jpg);
}