islacker

在发布文章中，如果文章中引用了远程图片，那么之前的做法就是：下载这个远程图片，然后重新上传。 
有了这个插件，你就可以省掉这些工作了，插件会在提交的时候，检查文章中引用的图片，如果是远程图片，就自动下载到附件目录，并保存到媒体库。 
如果没有选择特色图片，会自动选择第一张下载的图片作为特色图片。

Sync Remote Images

这个一个能在发布文章时同步文章中的远程图片的插件。

prebhdevsingh

The purpose of this plugin is very simple – when a post is published, it will fetch the first remote/external image that is referenced. The image that is retrieved is then attached to the post as the featured image. There are no settings to configure.
Please do not use this plugin to violate copyrights.
Sample uses: You maintain multiple blogs; copy paste the post contents from one blog to the other and this plugin will automatically save the first external image to the second blog. There is no need to re-upload images to multiple sites.
This plugin can also retrieve and save images from sites like Flickr, Facebook, Picasa, etc.

Auto Save Remote Image

This plugin automatically downloads the first remote image from a post and sets it as the featured image.

Sync Remote Images vs Auto Save Remote Image