michele.settembre

This plugin was created to make easier the life of people that want to get pictures, images, photos from the web, rather than download your image on your computer and, then, upload it in your media library, you can paste the image url in the field and get it from the web straight in your media library. Afterword you can click to set the post thumbnail and you will find the aquired image in your media library, ready to be inserted in your post.
This plugin is a simple box, activate the plugin, follow the settings link, activate the option to see the new form in the post thumbnail metabox in the post edit page and it’s done!
I created this plugin mainly for me, when I was creating websites I incurred the need to quickly grab some of my photos from another website ands that was the reason of this idea, it is something like creating a tunnel from one site to another to transfer images.

Post Thumbnail from URL

Post Thumbnail From URL is a plugin to quickly import images from the web straight to your Media Library using a public URL.

nikita.global

NG-Lazyload plugin replaces all the thumbails and images in the content with the smallest image possible (1 pixel gif) and then shows the full image only when it is in viewport. 
The only plugin which also works with background images in style tag

NG-Lazyload

well done

Enables 'lazyload' for all thumbnails and images in the content. Developed by Nikita Menshutin https://nikita.global/

Post Thumbnail from URL vs NG-Lazyload