How Do I Add a Pop-up Photo Thumbnail in a Post?

Apparently, I missed the option or button to add an image to my Classified Post that will pop-up in the corner when one hovers over the linked thread. There is a photo in the post but it does not appear in the pop-up; it seems that there is a default because I can't identify the image that's there... Can you add this later after the thread is posted?

Thanks...

Philip