Windows 'knows' the size in pixels for a Bmp:
$sFilename = "Ch.BMP"
$sFolderPathspec = "L:\"
Local $Object = objCreate("Scripting.FileSystemObject")
Local $objShell = objCreate("Shell.Application")
Local $objFolder = $objShell.Namespace($sFolderPathspec)
$strPixsize = $objFolder.GetDetailsOf($objFolder.Parsename($sFileName), 31)
msgbox(64,"size in pixels",$sFolderPathspec & $sFilename & @CrLf & $strPixsize,11 )
;
Exit
;
; the script works with a BMP, JPG, PNG, TIF, GIF, ICO file