Auto Thumbnails will help you upload Feature Image.

A simple plugin for theme developers to dynamically generate thumbnails on the fly without having to ask the users to manually regenerate them.

Corrects http image links on a page delivered over https.

Thumbnail RecentPost is widget

Video Thumbnails simplifies the process of automatically displaying video thumbnails in your WordPress template.

This Plugin helps to create thumbnails of uploaded images

Lazy Load for WordPress images bosst your website loading speed. Load images only when required. Show images than after scrolling down and when viewport and Lazy Load Background Images loads then Improved page speed.