Hi...TheOracle thanks for you answer but that's not I'm looking for, finally I did that modification myself.
Open admin/validateimages.php
1.1 Find:
$sql = "SELECT i.image_id, i.cat_id, i.user_id, i.image_name, i.image_date, i.image_media_file".get_user_table_field(", u.", "user_name")."
Replace with
$sql = "SELECT i.image_id, i.cat_id, i.user_id, i.image_name, i.image_date, i.image_media_file, i.image_thumb_file".get_user_table_field(", u.", "user_name")."
1.2 Find:
$file_src = get_file_path($image_row['image_media_file'], "media", 0, 1);
Replace with
$file_src = get_file_path($image_row['image_thumb_file'], "thumb", 0, 1);
Now save and upload the file.
Done.
Jemestave.