add_action( 'genesis_entry_content', 'sk_opening_image_wrap', 7 );
add_action( 'genesis_entry_content', 'sk_closing_image_wrap', 9 );

function sk_opening_image_wrap() {
    if ( ! ( is_home() || is_archive() || is_search() ) ) {
        return;
    }
    echo '<div class="one-third first">';
}

function sk_closing_image_wrap() {
    if ( ! ( is_home() || is_archive() || is_search() ) ) {
        return;
    }
    echo '</div>';
}

add_action( 'genesis_entry_content', 'sk_post_content_opening', 9 );
add_action( 'genesis_entry_content', 'sk_post_content_closing', 11 );

function sk_post_content_opening() {
    if ( ! ( is_home() || is_archive() || is_search() ) ) {
        return;
    }
    echo '<div class="two-thirds">';
}

function sk_post_content_closing() {
    if ( ! ( is_home() || is_archive() || is_search() ) ) {
        return;
    }
    echo '</div>';
}