How to remove/replace default image on every blog post

The blog aggregate page has pulled a default photo (from somewhere) that it uses on every post. The same pixelated picture shows up under the title of every post. I tried going to the template (I think…) and unchecking "featured image" but it's still there.

How do I get rid of this image, replace it with an image under the titles, and get the aggregate page to pull an image from the actual post?