How to add a credit line to a photo caption

I tried to do this once and almost got almost. Here’s what I did learn. Depending on your needs, this solution may be workable.

You can append content to media when it’s inserted into the editor using the get_image_tag filter:

/**
 * Filter to get caption and append image information to it.
 *
 * @return text HTML content describing with media attribution
 **/

add_filter('get_image_tag', 'my_append_media_credit', 20, 2);

function my_append_media_credit( $html, $id ) {

    $my_media_string = /* get your fields and put them together nicely */;

    $html = $html . $my_media_string;

    return $html;

}

However, you’ll notice that I mentioned it only works for images when they’re inserted into posts (and possibly updated, though I’d need to check out that). So if you’re starting a site from scratch, that may get you far enough, but I got stuck on retroactively applying it to existing images in posts.

You could get some additional coverage for images with captions by filtering img_caption_shortcode which filters the shortcode output but that still leaves images without captions and already in posts uncovered.

UPDATE 10/23/13 2:20pm PST:

Here’s the function I used to generate the $my_media_string in the code above:

/**
 * Function to lookup media creator fields and put them together.
 *
 * returns an html string with creator name and license name links.
 **/

function my_media_credit_string( $my_media_id ) {

    $my_name = get_post_meta( $my_media_id, 'my_creator_name', true );
    $my_name_url = get_post_meta( $my_media_id, 'my_creator_url', true );

    $my_creator = null;
    if( $my_name ) {
        if( $my_name_url ) {
            $my_creator=" <a href="" . $my_name_url . '">' . $my_name . '</a>.';  
        } else {
            $my_creator=" " . $my_name;
        }
    }

    return '<span class="my-media-attr">Credit:' . $my_creator . '</span>';

}

So the first snippet would get this:

    $my_media_string = $my_media_credit_string( $id );