gistfile1.php

<?php
/**
 * Change "Featured Image" to "Achievement Image" throughout media modal.
 *
 * @since  1.3.0
 *
 * @param  array  $strings All strings passed to media modal.
 * @param  object $post    Post object.
 * @return array           Potentially modified strings.
 */
function badgeos_media_modal_featured_image_text( $strings, $post ) {

	if ( badgeos_is_achievement( $post->ID ) ) {
		$strings['setFeaturedImageTitle'] = __( 'Set Achievement Image', 'badgeos' );
		$strings['setFeaturedImage'] = __( 'Set achievement image', 'badgeos' );
	}
	
	return $strings;
}
add_filter( 'media_view_strings', 'badgeos_media_modal_featured_image_text' );

/**
 * Change "Featured Image" to "Achievement Image" in post editor metabox.
 *
 * @since  1.3.0
 *
 * @param  string  $content HTML output.
 * @param  integer $ID      Post ID.
 * @return string           Potentially modified output.
 */
function badgeos_featured_image_metabox_text( $content, $ID ) {
	if ( badgeos_is_achievement( $ID ) )
		$content = str_replace( 'featured image', __( 'achievement image', 'badgeos' ), $content );
	
	return $content;
}
add_filter( 'admin_post_thumbnail_html', 'badgeos_featured_image_metabox_text', 10, 2 );

/**
 * Globally replace "Featured Image" text with "Achievement Image".
 *
 * @since  1.3.0
 *
 * @param  string $string Original output string.
 * @return string         Potentially modified output string.
 */
function badgeos_featured_image_metabox_title( $string ) {
	
	// If this is a new achievement type post
	// OR this is an existing achievement type post
	// AND the text is "Featured Image"
	// ...replace the string
	if (
		(
			( isset( $_GET['post_type'] ) && in_array( $_GET['post_type'], badgeos_get_achievement_types_slugs() ) )
			|| ( isset( $_GET['post'] ) && badgeos_is_achievement( $_GET['post'] ) )
		) && 'Featured Image' == $string

	)
		$string = __( 'Achievement Image', 'badgeos' );
	
	return $string;
}
add_filter( 'gettext', 'badgeos_featured_image_metabox_title' );