How do I get a random image from subset of images in media gallery?

Question

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.

Answer 1

You can use taxonomies with the media library, you just need to either add a built-in taxonomy, or register a new one.

For example, adding the built-in post tag taxonomy to attachments:

function wpd_attachment_taxonomy() {
    register_taxonomy_for_object_type( 'post_tag', 'attachment' );
}
add_action( 'init', 'wpd_attachment_taxonomy' );

You can then query for images with a specific tag:

$args = array(
    'post_type' => 'attachment',
    'post_status' => 'inherit',
    'tag' => 'tagged',
    'posts_per_page' => 1,
    'fields' => 'ids',
    'orderby' => 'rand'
);

$image = new WP_Query( $args );

if( $image->have_posts() ){
    $image_attributes = wp_get_attachment_image_src( $image->posts[0], 'full' );
    echo $image_attributes[0];
}

Change tagged to whatever the slug of your chosen tag is. This will get the attachment ID of one random image with your tag. We can then use that ID to get the attachment image attributes.