Recent Posts Widget with Thumbnails

Kale also integrates with the  Recent Posts Widget With Thumbnails plugin. Assign the widget to any sidebar location to show recent posts from any category (or multiple categories) along with their featured images. 

Make sure to select “kale-thumbnail (760×400)” as the image size to display and also check on the “Use aspect ratios of original images? “ option (which makes sure that the images are displayed in a responsive fashion).