Rename “Add Media” Button To “Add Images”

The button text being a translatable string, you can make use of the gettext filter:

function wpse95025_rename_media_button( $translation, $text ) {
    if( is_admin() && 'Add Media' === $text ) {
        return 'Add Images';
    }
    return $translation;
}
add_filter( 'gettext', wpse95025_rename_media_button, 10, 2 );

For the sake of completeness:
Of course, you can also keep the new string translatable.

function wpse95025_rename_media_button( $translation, $text ) {
    if( is_admin() && 'Add Media' === $text ) {
        return __( 'Add Images', 'your-text-domain' );
    }
    return $translation;
}
add_filter( 'gettext', wpse95025_rename_media_button, 10, 2 );