/* Hide all by default */
.banner-img {
  display: none;
}

/* Show for specific categories */
.category-id-3 .banner-cat-3 {
  display: block;
}
.category-id-7 .banner-cat-7 {
  display: block;
}

/* Show default for all others */
body:not(.category-id-3):not(.category-id-7) .banner-default {
  display: block;
}