Manually deleting images (from db and file_usage table)
This is probably more of a comment but the code won't fit in...there should be no reason for the UI to fail on the file delete, unless the web server daemon doesn't have write permission to the files folder. That would be the first thing I'd check, then try again.

If that fails, you might want to give it a go manually:

$nids = db_query("SELECT nid FROM {node} WHERE type = 'the_type'")-&gt;fetchCol();

foreach (node_load_multiple($nids) as $node) {
  $node-&gt;field_name[LANGUAGE_NONE] = array();
  node_save($node);
}


That will take care of the whole lot (removing the data from file_managed, file_usage, and deleting the file from the disk if it exists and is writeable).