I think I’ve answered my own question –
Switch the order of where the gallery appears on the page to after where the link appears, i.e:
<a href="/link-to/image.jpg" rel="[lightbox_set20]"><img class="aligncenter wp-image-1565" title="Read Here" src="/link-to/image.jpg" alt="" width="320" height="305" /></a>
<div style="display: none;">[nggallery id=20]</div>
Then, make the href=”” URL an image in the gallery, and the img src=”” can be any image you want the thumbnail link to be. To prevent a duplicate image from appearing in your gallery, remove whatever image you’re using as the starting image set as the href=”” url from your gallery.
