I want to share an external link (news article) to my company's Facebook page, but the thumbnail is just a generic image showing the name of the news site, so it looks like I'm advertising the newspaper. Is there a web-app or browser extension that allows me to change the preview image to something more relevant to the topic of the article?
You would need to change the meta tags of the page you're sharing. If you can't do this (because you don't have access to the site's HTML code or you're sharing an external page), you could try ShareKit.io, which is a web app for changing the Facebook link preview. It also works for other social platforms like Twitter and LinkedIn, as well.