next & previous posts

Hi,

In the sidebar of a post mi visitor can see next & previous.
The links to the posts are working, but there are no images of the blog-posts…
(https://gluecksschmie.de/blogparade-was-ist-geld/)

How can I make it possible to see pictures and how could I switch off that service of next & previous posts?

Thank you in advance,
Martin