You can add your own columns to the list of posts, pages or custom post types in the dashboard, for example a thumbnail, a price or a custom field. Two hooks are needed: one to register the column and one to print its content.

Add a thumbnail column to a custom post type

The hook names contain the post type. For regular posts use manage_posts_columns and manage_posts_custom_column, and for pages use manage_pages_columns and manage_pages_custom_column.

function lotgrafix_portfolio_columns( $columns ) {
    $new = array();

    foreach ( $columns as $key => $label ) {
        $new[ $key ] = $label;

        if ( 'cb' === $key ) {
            $new['lotgrafix_thumb'] = __( 'Image', 'lotgrafix' );
        }
    }

    $new['lotgrafix_client'] = __( 'Client', 'lotgrafix' );

    return $new;
}
add_filter( 'manage_lotgrafix_portfolio_posts_columns', 'lotgrafix_portfolio_columns' );

function lotgrafix_portfolio_column_content( $column, $post_id ) {
    if ( 'lotgrafix_thumb' === $column ) {
        echo get_the_post_thumbnail( $post_id, array( 60, 60 ) );
    }

    if ( 'lotgrafix_client' === $column ) {
        echo esc_html( get_post_meta( $post_id, 'lotgrafix_client', true ) );
    }
}
add_action( 'manage_lotgrafix_portfolio_posts_custom_column', 'lotgrafix_portfolio_column_content', 10, 2 );

Remove an existing column

function lotgrafix_remove_columns( $columns ) {
    unset( $columns['comments'] );
    unset( $columns['tags'] );
    return $columns;
}
add_filter( 'manage_posts_columns', 'lotgrafix_remove_columns' );

Make a column sortable

function lotgrafix_sortable_columns( $columns ) {
    $columns['lotgrafix_client'] = 'lotgrafix_client';
    return $columns;
}
add_filter( 'manage_edit-lotgrafix_portfolio_sortable_columns', 'lotgrafix_sortable_columns' );

function lotgrafix_sort_by_client( $query ) {
    if ( ! is_admin() || ! $query->is_main_query() ) {
        return;
    }

    if ( 'lotgrafix_client' === $query->get( 'orderby' ) ) {
        $query->set( 'meta_key', 'lotgrafix_client' );
        $query->set( 'orderby', 'meta_value' );
    }
}
add_action( 'pre_get_posts', 'lotgrafix_sort_by_client' );

Style the column width

function lotgrafix_admin_column_css() {
    echo '<style>.column-lotgrafix_thumb{width:80px;}</style>';
}
add_action( 'admin_head', 'lotgrafix_admin_column_css' );