To center the “Follow” button over the images in the feed, add the following JavaScript to your site. This can be done manually using a custom JavaScript plugin. For further information see our documentation here.
JavaScript
$('#sb_instagram #sbi_images').prepend( $('.sbi_follow_btn') );
Then add the following CSS to WordPress Dashboard > Appearance > Customize > Additional CSS (or your preferred CSS location).
CSS
#sbi_images {
    position: relative;
}
.sbi_follow_btn {
    position: absolute;
    z-index: 2;
    top:50%;
    right:50%;
    margin-right: -85px;
    margin-top: -20px;
}
May need to adjust these numbers to perfectly center the follow button