One Crayon

Markup to Featured Image will automatically generate a featured image when you publish a post, with the intent of allowing you to publish posts with featured images from third-party clients like MarsEdit.
To add a featured image to your post, include one of the following markup variants in your post’s content:
<pre><code><img src="/path/to/image.jpg" data-featured-image="keep" />
<img src="/path/to/image.jpg" data-featured-image="strip" />

</code></pre>
If you use an <code><img></code> element, the values of the <code>data-featured-image</code> attribute specify whether the image should be displayed when the post is shown on your site (<code>keep</code>) or ignored when the post is displayed (<code>strip</code>). The order of attributes in your <code><img></code> tag (and whether you include other attributes, like width/height, title, alt, etc.) doesn’t matter. You can make it as complicated or simple as you like.
If the post already has a featured image, the plugin will do nothing (version 1.0 does not allow updating the thumbnail by updating the markup).

Markup to Featured Image

Automatically set the featured image (formerly the post thumbnail) for a post based on custom markup in the body text.

ryanshoover

Cousett

Easily display a mobile featured image on your posts and page!
Up to 70% of website traffic today is on mobile devices. Sometimes WordPress does not quite resize the featured image in the best way. Avoid this and have full control over your featured images on any device with this plugin.
To add a mobile featured image, simply upload a new image in the meta box right below the WordPress Featured Image called Mobile Featured Image. The new image can be a specifically resized version of your featured image or an entirely new image targeted especially for mobile viewers.
You can add your mobile featured image without worrying about slowing down your site. The mobile image is added to the <code>srcset</code> attribute of your featured image. Vistors’ browsers will automatically download the best image for the size of their screen.
This plugin works by filtering the <code>wp_calculate_image_srcset</code> function and changing the url for all screens under 980w to the mobile featured image. 980w is a reliable breakpoint between the iPhone 6+ and the iPad, two common measures for when a mobile image would be needed.

Markup to Featured Image vs Mobile Featured Image