I think you were pretty close, so I would cycle $i = 1 to $i = 5 with
$item_picture_id = $item['Item']['ItemPicture']['id'] //or something, im not sure how you have done your relations
echo $this->Form->postLink($this->Html->tag('i', '',
array('class' => 'glyphicon glyphicon-remove')),
array('controller' => 'item_pictures','action' => 'delete_current_img', $item_picture_id, 'img'.$i) ,
array('escape' => false), __('Are you sure you want to delete This Image?'));
So for example image 5 ($i = 5) should resolve to the URL: item_pictures/delete_current_img/ID/img5/
Then for your controller just have (very simple example):
public function delete_current_img($id,$field){
$this->ItemPicture->id = $id;
$this->ItemPicture->saveField($field,'');
//then just go back to where you come from
$this->redirect(array('controller' => 'foo', 'action' => 'where_ever_the_delete_came_from'));
}
Hope it helps!