Sitewide thumbnail for WP site without blog/posts page?

I just launched a new portfolio website that doesn’t have a blog/posts page. If I post a link to this website — on Facebook, for example — a random item from my portfolio images shows up as the thumbnail for my website. I would like to be able to set *one* thumbnail that will show up for all links to the site on social media platforms. Anyone know how to do this? Thank you!