How can i display portfolio images in WordPress?

First

Try this to get the thumbnails, $size default is ‘thumbnail’:

  wp_get_attachment_image( $attachment_id, $size, $icon, $attr );

Second

This code not tested with your code. Swap out the (taxonomy) for your tag.

Check out this page for more information about the columns you are building in your category page. Manage $taxonomy id columns

function my_custom_(taxonomy)_columns( $columns ) {

    $columns = array(

        'cb' => '<input type="checkbox" />', // must keep

        'image' => __( 'Image' ),

        'title' => __( 'Isim' ),

        'tanim' => __( 'Tanim' ),

        'yazikisaismi' => __( 'Yazi Kisa Ismi' ),

        'toplam' => __( 'Toplam' )

    );

    return $columns;

}

add_filter( 'manage_edit-(taxonomy)_columns', 'my_custom_(taxonomy)_columns' ) ;