Most Zen Cart templates don't provide the information Facebook, Twitter etc. need to find what you want to be the hero image for a product page. Facebook will thus just make an attempt at it, and will more than likely decide on any sort of image early on the page well before it gets to the image you think should be the hero image. I suspect that's what's happening in your case.
If you take a look at the source code to a page like https://www.scubadoctor.com.au/dives...oducts_id=3960 you'll see a bunch of meta tags in the <head> to tell Facebook what it should use.
Code:
<meta property="og:site_name" content="The Scuba Doctor Dive Shop" />
<meta property="fb:app_id" content="1379362135704555" />
<meta property="fb:admins" content="643082922,709231463,100000903418809,1761507821" />
<meta property="og:street-address" itemprop="street-address" content="1/49 Peninsula Ave" />
<meta property="og:locality" content="Rye" />
<meta property="og:region" content="VIC" />
<meta property="og:postal-code" content="3941" />
<meta property="og:country_name" content="Australia" />
<meta property="og:email" content="[email protected]" />
<meta property="og:phone_number" content="+61 3 5985 1700" />
<meta property="og:type" content="og:product" />
<meta property="og:title" content="Scuba Doctor Crayfish Season Professional Kit - Special Offer - The Scuba Doctor Dive Shop" />
<meta property="og:url" content="https://www.scubadoctor.com.au/diveshop/index.php?main_page=product_info&products_id=3960" />
<meta property="og:image" content="https://www.scubadoctor.com.au/diveshop/images/scuba_doctor/crayfish-season-2015-special-offer.jpg" />
<meta property="og:description" content="SPECIAL OFFER FOR THE CRAYFISH SEASON The 2015 Crayfish Season Pro Kit from The Scuba Doctor combines the favourite, tried and proven, products used by our local expert cray bashers: the brilliant Dive Perfect Stubby LED-1000 Dive Light for finding the crays, the magnificently durable Apollo Kevlar Commercial 3 mm Dive" />
<meta property="og:condition" content="NewCondition" />
<meta property="product:retailer_part_no" content="CRAY-PRO-2015"/>
<meta property="og:category" content="Scuba Packages" />
<meta property="product:price:amount" content="269" />
<meta property="product:price:currency" content="AUD"/>
<meta property="og:availability" content="InStock" />
Then there's more meta tags to tell Twitter what to use.
These meta tags are put there using an appropriate Zen Cart plugin.
Then Facebook, Twitter and other social meta sites that can read and use these meta tags do so. Then you get the hero image and other information you want highlighted.
Best regards, Lloyd Borrett.