tekitou

This forbid visitor to download images. 
That’s all.
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Image Protector

strawman sockpuppet

This plugin allow you to grab image from remote url and save into your own wordpress media library. By doing so, you never worried if the remote image was removed by its owner. This also save you steps to download the image to local computer and upload again to your own wordpress.
After grabbing the image, wordpress will prompt you either to “insert into post” or “change attributes” just like after you upload an image. Renaming is available.

Grab & Save

This plugin allow you to grab and save the image from remote url into your own wordpress media library.

Image Protector vs Grab & Save