Hi there,
Thanks for writing in! To remove the black box, you will need to use CSS. You can add this under Custom > CSS in the Customizer or in your child theme’s style.css file.
.x-recent-posts a {
background: transparent;
border: none;
box-shadow: none;
}
Regarding the image, you will need to go to the post itself, edit the post and add a featured image.
Let us know how that goes. Hope this helps – thanks!