Hi Sarah,
What if you do this?:
#wpsp-123 .wp-show-posts-inner .wp-show-posts-image img {
filter: grayscale(100%);
-webkit-filter: grayscale(100%);
}
#wpsp-123 .wpshow-posts-inner:hover .wp-show-posts-image img {
filter: none;
-webkit-filter: grayscale(0);
}Be sure to replace 123 with the ID of your list.