1.Log in to your dashboard--> layout- -> Edit HTML
2.Click on "Expand Widget Templates"
3.Now find "Read More" text in your template.
Note:It can be different from "Read More" according to your template.(Eg:more,full story,....)
4.Now replace it with below code:
Note: Replace "LINK-OF-YOUR-IMAGE" with your image link.
You are done.