Programmatically delete CCK field value
You will need to check the ImageField array to see what needs to be changed but for D6 but it   should be something like the below:

$node = node_load($nid);
$node-&gt;field_photo[0]['value'] = NULL;
node_save($node);


Just change the field name to what you are using and change the $nid to the node id #.