I decided to try an add a field using what you did. I never did get the problem you are having, and I worked from your template. Make sure you go to line 232 of plog-manage and update it:
case 'update-picture': // Update the picture information if (!isset($_REQUEST['cancel'])) { $allow_comments = (isset($_REQUEST['allow_comments'])) ? $_REQUEST['allow_comments'] : ''; $action_result = update_picture($_REQUEST['pid'], $_REQUEST['caption'], $allow_comments, $_REQUEST['description'],$_REQUEST['precio']);