How To Set A Custom Sharing Image (Thumbnail) For A Squarespace Page
One major complaint about Squarespace, especially when working with Index Pages and pages with banners, is the lack of flexibility in setting a social sharing image. If the page you're sharing has a banner it is automatically set as the sharing image, and can't be easily overridden in the editor.
However, ONE line of code can add a custom image. And here it is:
<meta property="og:image" content="imageurl">
The meta property og:image is what social media sites like Facebook pull as the sharing image. And if you add this line of code to the page header (found in the settings of the individual page under the Advanced tab), inserting an image URL like this one:
<meta property="og:image" content="https://static1.squarespace.com/static/56b68c77a3360c432ba99425/59ad0f70f5e23155dc8fb2bd/59ad121de3df2822f08913ef/1504514589905/IMG_0645.PNG">
It will set your new sharing image!
If you have already previous shared the page you'll want to have social sites scrape the information again, using a tool like the Facebook Debugger or Twitter Card Validator. All you need to do is paste your page link (make sure it's the front-end link, not the .squarespace.com link) and click debug or preview to see your new image.