How do I force an image to use when a website is shared?
You can specify a specific image to use for URLs sent in iMessage by using a meta tag with og:image set to the property attribute. Apple recommends your image being at least 900px wide or larger.

Example:

&lt;meta property="og:image" content="https://www.example.com/image.png" /&gt;


I got this information from Apple’s Developer Documentation Archive.