Customize sidebar widget recent posts

Hi, I'd like our recent posts sidebar widget content to look like this one: https://www.dropbox.com/s/gd6f76j0jegfvf3/new%20sidebar%20design.jpg?dl=0 Meaning, the feature image should be the background, and the title should be shown on top of a gray overlay. How do I achieve this with CSS? Thanks a lot for your help, Michaela