[Creat0R]

Можно ещё так (с возможностью указать фиксированную ширину):

Код:

#include <GDIPlus.au3>

_Image_Scale('Src.jpg', 'Dst.jpg', 400)

Func _Image_Scale($sSrc_File, $sDest_File, $iWidth, $iHeight = -1)
    _GDIPlus_Startup()

    Local $hImage = _GDIPlus_ImageLoadFromFile($sSrc_File)
    Local $iImg_W = _GDIPlus_ImageGetWidth($hImage)
    Local $iImg_H = _GDIPlus_ImageGetHeight($hImage)

    Local $iScale, $hResized

    If $iHeight = -1 Then
        $iScale = (100 / $iImg_W * $iWidth) / 100
        $hResized = _GDIPlus_ImageScale($hImage, $iScale, $iScale)
    Else
        $hResized = _GDIPlus_ImageResize($hImage, $iWidth, $iHeight)
    EndIf

    _GDIPlus_ImageSaveToFile($hResized, $sDest_File)

    _GDIPlus_imageDispose($hImage)
    _GDIPlus_ImageDispose($hResized)

    _GDIPlus_Shutdown()
EndFunc