We don't have a built-in way that would automatically detect and set the icon for you. However, if you're creating the topic, you can always add the following tag right into the post to set the Open Graph image for the page to whatever you want:
{SCOpenGraph image=http://yoursite.com/yellow-icon.png}
I hope that helps get you going, but if you need anything else, just let us know.
Thanks,
Alex