Hmm. That's weird. For some reason the file isn't uploaded correctly.
If you have PHP 4.2.0+ you could add this to the error output: {$_FILES['Image']['error']}
If not, you could at least check if the filename is set by adding this to the error output: {$HTTP_POST_FILES['Image']['name']}
Otherwise I'm stumped... You haven't turned off file uploading in php.ini or set the max_file_size to 0 or something?