SEO and Photos

Sometimes I get double photos on my blog posts. SEO says to include a photo on the page and somewhere something asks you to set a “foundation photo”. I’m not sure if i’m using the right terminology for that. If I don’t set a foundation photo, then no photo appears on the blog master page with a snippet of the blog post. How can I prevent the double image when someone clicks a blog post and goes to the specific page where the blog post resides?