Here are the directions to find the Feature Image button
1) When editing a post, scroll to the bottom of the page and look on the right hand side.
2)You'll see some text there that you can click on that says "Featured Image."
3) Click on the text then select the photo that you want to feature. (Feature means show in the thumbnail or the slider.)
A feature imge is used for teh slider or a thumbnail. If you want the photo (and an embedded link to the post) to show up in the slider, you'll also need to "sticky" the post. To do that
1) When editing a post, look on the right hand side. You'll see something that says Visibility: Public [Edit].
2) Click the edit button
3) Tick the box that says "Stick this post to my front page"
Hope this helps!