Wells template: need varying image shapes in thumbnails

In my portfolio pages, I prefer the visiter to see an entire collection of work at a glance.  Is there a way to alter the thumbnail ratio so that square and rectangular, vertical and horizontal images show correctly? Right now, I have them set to square since most of my paintings are square, but the rectangular pieces get cropped.  Is there a different gallery option in Pages that would solve this issue easily? I have seen others who use Wells achieve the proper ratios for their artwork on what looks like a page set to thumbnails.

Thanks in advance for any help.