When you have thousands of products you feel the need to organize your images by something (e.g. the manufacturers name). But resize process do not work with sub dirs.
adding someting like:

      preg_match("/(.*)\/(.*)/i",$image,$subdir);
      if (!empty($subdir[1])) { $subdir[1] = '/'.$subdir[1];}
      if (!file_exists(DIR_FS_CATALOG . DIR_WS_IMAGES . 'products/' . $this->_groups[$group_id]['code']. $subdir[1])) {
        mkdir(DIR_FS_CATALOG . DIR_WS_IMAGES . 'products/' . $this->_groups[$group_id]['code'] . $subdir[1]);
        @chmod(DIR_FS_CATALOG . DIR_WS_IMAGES . 'products/' . $this->_groups[$group_id]['code'] . $subdir[1], 0777);
      }
  
to the resize function could be helpful.
Something should be changed in products_edit too(to create the subdir and save the image in it).

* Ticket imported from Trac. Original reporter: pknet