Making Pic of my post appear in bloggers friend blogroll.

Hi there, sorry in advance for being such a noob but I was wondering how my blog pic (from my posts) appears on my blogger friends post page. Like when I click on my friends' blog that has subscribed to my blog, I see my name, how long ago i submitted the post, the title of my post but theres no mini thumbnail picture of a pic on my post- whereas other bloggers have a picture next to the title of their post in my friends' blogroll. TIA