Facebook and Twitter utilize the og:image meta tag to populate a suggested image from a website; so simply link a smaller image there via the header includes section of the UBB.threads Control Panel (CP -> Display -> General -> HTML Includes -> "Header Insert for All Pages"):
<meta property="og:image" content="https://example.com/images/image.png">

Without this code their system will attempt to use the first image on the page (occasionally I've had it choose other images on the page).