Just add the following code in the Custom CSS or the stylesheet of your child theme and you are done.
For all images : featured page images and featured images in post list
.round-div { -webkit-border-radius: 0; -moz-border-radius: 0; border-radius: 0; }
Only for featured pages
.widget-front .round-div { -webkit-border-radius: 0; -moz-border-radius: 0; border-radius: 0; }
Only for featured images of posts
.article-container .round-div { -webkit-border-radius: 0; -moz-border-radius: 0; border-radius: 0; }
More about the border radius property.
More about Featured page customizations.