Sheri Grey

The <code>[random_images]</code> shortcode will display an unstyled set of random images.
Six images will display at thumbnail size linked to attachment pages by default.
To change the number of images and the size:
<pre><code>[random_images total=4 size=medium]
</code></pre>
To make the links point to image files instead of attachment pages:
<pre><code>[random_images link=file]
</code></pre>
If you have any questions, please ask in the <a href="https://wordpress.org/support/plugin/random-images" title="Random Images Support Forum" rel="ugc">support forum</a>.

Random Images

The [random_images] shortcode displays random attached images.

Brianna Deleasa

This plugin allows you to upload a set of images via a settings page to be shown for posts without images. The images will be chosen at random on page load for any posts without a featured image (meaning each time the page is reloaded another random image will be chosen). Useful if your theme shows thumbnails for the posts, and you don’t want any posts without thumbnails.
UPDATE: Now you can select images specifically for certain post types as well as taxonomy terms.

WP Random Post Thumbnails

Allows you to select images to be shown at random for posts without a featured image.

Random Images vs WP Random Post Thumbnails