I feel like this has to have been done a lot, and there are great plugins out there for it, but if you’re just looking to add a quick function to your theme (or a really simple plugin) yourself, here’s how to modify WordPress’ search query to include attachments (like images).
function attachment_search( $query ) {
if ( $query->is_search ) {
$query->set( 'post_type', array( 'post', 'attachment' ) );
$query->set( 'post_status', array( 'publish', 'inherit' ) );
}
return $query;
}
add_filter( 'pre_get_posts', 'attachment_search' );
This does 2 things: includes posts of type ‘attachment’ to the search query, and adds the post status of ‘inherit’. This will ensure that any images (or other attachments) that were added while adding a new post or page will be included in the results.
You can also extend the post type array to include your own Custom Post Types (eg array( 'post', 'attachment', 'products' ); )
These 2 posts were very helpful in getting this code together: