When you share forum topics or forum posts on social media sites such as Facebook and Twitter will use our default social share image or alternate social share image which doesn’t work well as representation of specific forum discussions.
Forum Topic Attachment
We have implemented a new feature which lets you use the first image uploaded as an attachment in a topic and set it as Open Graph image ("og:image"), so social media websites such as Facebook, Twitter, and Google+ can use it as a thumbnail image to represent the link.
This feature only adds the attachment image to the source code so that social media services can pick it up. Please note that there is no guarantee that the attachment will not instantly show up as social media websites do not scrape content automatically.
Facebook Sharing Debugger
Facebook caches the images and loads them asynchronously which means that when you share an URL for the first time, the image might not show instantly. If you want to test what Facebook is picking up, go to https://developers.facebook.com/tools/debug/sharing/ and re-scrape the URL.
If you don’t see an image for your topic URL or it is an outdated one, click “Scrape again” until it displays correct image.