Plugin for ImageResizer for .NET to generate a thumbnail image for a folder on disk using a randomly-selected image from that folder.
When a request comes in for an image with one of the specified filenames, if that image does not exist on disk then an image is chosen at random from the same directory and a thumbnail of it is served instead. If there are no images in the folder then a default fallback is used, if specified, as a last resort.
Add something like the following to the resizer section in web.config to specify one or more virtual filenames to activate the thumbnail generator and a default fallback.
<folderThumb default="default-folder-thumb.jpg" activateOnFilename="folder.jpg,thumbnail.jpg" />