I would use something like this :
Code:
alias tryit {
  if (!$exists($1-)) .timertry -m 0 20 if ($dialog(try)) $({,0) did -h try 1 $(|) .timertry off $(},0) 
  return $iif(!$v1,$1-,$mircexe)
}
dialog try {
  title try
  size -1 -1 300 300
  option dbu
  icon 1, 10 10 10 10, $tryit(path/file.ext)
}
If the file does not exists, just use the first mirc icon file, and hide the icon as fast as possible (i use -m 0 20, because it's fine, but you can change these value)

#mircscripting @ irc.swiftirc.net == the best mIRC help channel