Use thumbnail instead of resize full size image

Hi, I like your plugin but I have an issue. I have the widget setup to grab the feature image of a post but it uses the full size picture for the thumbnail and just scales it down to a thumbnail size. I have a lot of images in my widget so this is a huge waste of bandwidth and make the page slow because the visitor loads all the full size images at once when they visit the page. Is there a way for the widget to grab a thumbnail size image instead of rescaling the full size one?

https://www.ads-software.com/extend/plugins/advanced-random-posts-thumbnail-widget/