Actually forget the DLL my mistake, i had really forget that, Loki is right, mIRC has this feature by default on dialogs, just put the "test.ico" into your mirc.exe directory and use this code:

To run the dialog: /dialog -m test test

Code:
dialog test {
  title "Test Dialog"
  size -1 -1 233 121
  option dbu
  icon test.ico, 0
  button "Button", 1, 97 105 37 12
}

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