Had the same issue , I finally got it working after some trying. Here are the 8 html tags that I used on my web page to get the preview working :
In <head>
tag:
<meta property="og:title" content="ABC Blabla 2020 Friday" />
<meta property="og:url" content="https://bla123.neocities.org/mp/friday.html" />
<meta property="og:description" content="Photo Album">
<meta property="og:image" itemprop="image" content="https://bla123.neocities.org/mp/images/thumbs/IMG_327.JPG"/>
<meta property="og:type" content="article" />
<meta property="og:locale" content="en_GB" />
In <body>
tag:
<link itemprop="thumbnailUrl" href="https://bla123.neocities.org/mp/images/thumbs/IMG_327.JPG">
<span itemprop="thumbnail" itemscope itemtype="http://schema.org/ImageObject">
<link itemprop="url" href="https://bla123.neocities.org/mp/images/thumbs/IMG_327.JPG">
</span>
These 8 tags ( 6 in head , 2 in body) worked perfectly.
Tips:
1.Use the exact image location URL instead of directory format i.e. don't use images/OG_thumb.jpg
2.Case sensitive file extension: If the image extension name on your hosting provider is ".JPG" then do not use ".jpg" or ".jpeg' . I observed that based on hosting provider and browser combination error may or may not occur, so to be safe its easier to just match the case of file extension.
3.After doing above steps if the thumbnail preview is still not showing up in WhatsApp message then:
a. Force stop the mobile app ( I tried in Android) and try again
b.Use online tool to preview the OG tag eg I used : https://searchenginereports.net/open-graph-checker
c. In mobile browser paste direct link to the OG thumb and refresh the browser 4-5 times . eg https://bla123neocities.org/nmp/images/thumbs/IMG_327.JPG