If a custom post does not have a featured image, I would like to grab an image from the media library to use in place of the featured image. These particular images wouldn't be attached to any particular post, and would only be used in place of a featured image in posts where there is none. I don't want to get any image from the media library, only ones that I've somehow tagged.
I have it working now with a set of hardcoded images (that I just ftp'd to the uploads directory). However, I would like the ability to just add images to the media library and somehow "tag" them that they are for use as a default image. Since there doesn't appear to be categories/tags/taxonomies for uploaded images, what would be the best solution to get this functionality?
The only thing I could think of is create a page, and upload all my "default images" to that page. Then just query for all the images attached to that page and use a random one. There has to be a better way though than creating a useless page to categorize images.