WordPress allow use of html img tag in the comments, so the images are shown directly if wrapped with it, but most of the people who visits our blogs do not know how to use html tags. After unsuccessful searching for instant solution, I decided to build a plugin which exactly do that.
This plugin is simple. It uses preg_replace to detect direct image links in comments and wraps those with the img html tag. On top of it, you can specify width of your comment so it creates container and then fits image into it. It does not resize image it just puts image of any size into specified container. This is good because nothing is stored on your server, but bad because people can link images of virtually any size which will slow loading of your page for your visitors.
I am planning in next version to detect size of the image on given link and if it is over X megabytes and x*y pixels it skips wrapping image in IMG tag.
Currently this plugin embeds only images with .jpg, .png and .gif extension, but I am planing making interface so you can specify which image types should be embedable.
You can download plugin from WordPress.org or here:
Embed Comment Images DEMO
You can use this post as a testing demo. Just copy/paste the image link directly into the comment field.
p.s.
If you want to embed video content URLs in the comments, you can do so by using oEmbed plugin.