Images with non-ASCII characters in their filenames cannot be converted.

This is due to a limitation/bug in PHP, where escapeshellarg() removes some non-ASCII characters in certain cases. I've found that the following manual replacement works better (though it's horribly ugly):

/**
 * A non-ASCII-character safe replacement of escapeshellarg().
 */
function escapeshellarg_special($file) {
  return "'" . str_replace("'", "'\"'\"'", $file) . "'";
}

Not sure if there are other implications to this, but it's something to be considered. This module may be unusable for some countries without using the transliteration module or an ugly workaround like this.