Hi, Jonathan.

You can add customization for the function youplay_post_thumbnail in child theme.

Original function code:

if ( ! function_exists( 'youplay_post_thumbnail' ) ) :
/**
 * Prints HTML with image and post url
 */
function youplay_post_thumbnail( $get = false, $additional_info = '' ) {
    $template =
        '<a href="%s" class="angled-img">
            <div class="img">
                %s
            </div>
            ' . $additional_info . '
        </a>';
    if ( has_post_thumbnail() ) {
        $thumbnail = get_the_post_thumbnail( get_the_ID(), '500x375' );
    } else {
        $thumbnail = '<img src="' . esc_url(yp_opts('single_post_noimage')) . '" alt="no image">';
    }
    if($get) {
        return sprintf( $template, esc_url( get_permalink() ), $thumbnail );
    } else {
        printf( $template, esc_url( get_permalink() ), $thumbnail );
    }
}
endif;

You can activate child theme and insert this function in functions.php file. Then you will be able to change thumbnail size. See this part of code:

$thumbnail = get_the_post_thumbnail( get_the_ID(), '500x375' );

In the next update will be added possibility to change image resolution in theme options.

Best regards, nK.