This should be what you're looking for:

Code:
alias icon { return $mircdir\img\icon\ $+ $$1 }
[color:Red][/color]
dialog ico {
  title "Hover Icon"
  size -1 -1 75 20
  option dbu
  icon 1, 1 1 17 18, $icon(Icon_1.ico), 0
  button "Close", 2, 25 1 50 18, cancel
}
[color:Red][/color]
on 1:DIALOG:ico:MOUSE:*: {
  ; Un comment me to find out where locations on your dialog are (used to reference
  ; where the mouse should be when the icon is to change)
  ; /echo -s Mouse: $mouse.x $mouse.y
  if (($mouse.x isnum 2-35) && ($mouse.y isnum 2-35))  {
    if ($did(1) != $icon(Icon_2.ico)) /did -g $dname 1 $icon(Icon_2.ico)
  }
  else {
    if ($did(1) != $icon(Icon_1.ico)) /did -g $dname 1 $icon(Icon_1.ico)
  }
}