Kay Joe

Important Aspects
<ul>
<li>The default featured image will show exactly how your theme shows it. That is if it appears besides the excerpt it will show there. It will not show on top of the excerpt or misaligned, as this plugin only changes the image and does not add any html attributes.</li>
<li>This simple plugin sets an image that has been already uploaded in the WordPress Media Library as the default Featured Image for all posts that do not have a featured image already set.</li>
<li>This plugin can also remove or unset an image that exists in media library and is set as featured image for one or more posts. This is the remove default featured image feature.</li>
<li>Your theme must already support post thumbnails for this plugin to work. This plugin will show the default featured image you set in same style and position as shown by your active theme. Only the image will be changed</li>
<li>If you are using a cache plugin, try disabling it and clear browser cache to see results. You can always go to a post and see whether the featured image has been set or unset.</li>
<li>Please deactivate any other featured image plugin before using this plugin. Also, if you have modified the code in your theme files to show a default featured image then please delete that code, else plugin will have problems</li>
<li>The following steps are recommended but not necessary.These may be required to set the size for the default featured image.
<ul>
<li>To Set a default featured image for a post, check the size of the featured image that your theme is showing. This can be dome by right click on the image->Choose view Image OR copy its link or address and open in new browser. Right click on the image and see its dimensions in image information.</li>
<li>If your theme’s functions.php file uses the add_image_size function and the size of the image uploaded to media library to be used as default featured image, is higher than the max size set for thumbnail by any of the _add_image_size_then the selected image to be set as default image will be resized and saved as a new image with size as the maximum size set to be used as thumbnail (not single post) by those functions.</li>
<li>It is recommended that the Settings of the small thumbnail in the Settings-Media and the size set by add_image size be kept same
<ul>
<li>After that please resize the image to be set as default featured to the smaller size (to optimize page load time) or resized exactly as the dimensions of smallest size in Settings->Media as set by the add_image_size function</li>
<li>Then uploaded the image to the media library and set it as default featured image using this plugin</li>
</ul>
</li>
</ul>
</li>
</ul>
<h4>Suggestions are welcome</h4>
<ul>
<li>Found a bug?</li>
<li>Something to be improved?</li>
</ul>
Contact me @kayjoe on WordPress.org or ano[dot]shani[at]gmail[dot]com

Default Featured Image Themed

Set an uploaded image as a default featured image for all posts without a featured image. Remove an image set as featured image for one or more posts.

Sergey Biryukov

Allows to find all posts where a particular attachment (image, video, etc.) is used.
Adds a “Used In” column to the list view in Media Library, as well as to the attachment details modal.
Finds posts that use the image (including any of its intermediate sizes) in post content, or as a featured image. Works with custom post types too.
Inspired by a <a href="http://wptavern.com/the-problem-with-image-attachments-in-wordpress" rel="nofollow ugc">WP Tavern post</a> on image attachments.

Find Posts Using Attachment

Allows to find all posts where a particular attachment is used.

Default Featured Image Themed vs Find Posts Using Attachment