Random image from tag/custom type on sidebar

If I get your question right, as you mention Tag in the title, but not in the Content – and also mention a random image but then it’s not a library image but a custom field value…

So, the following code will grab 1 random custom post type that has a custom field value that’s not empty. And it’s a mix of get_posts, get_post_meta and Custom_Field_Parameters:

$args = array(
   'numberposts' => 1,
   'orderby' => 'rand',
   'post_type' => 'athletes',
   'post_status' => 'publish',
    'meta_query' => array(
        array(
            'key' => 'athletethumbnail',
            'value' => '',
            'compare' => '!=' /* will not grab posts with empty Custom Field */
        )
    )

);
$show_albums = get_posts ( $args );
echo get_post_meta($show_albums[0]->ID,'athletethumbnail',true);