hBitmap := LoadPicture(imageFilePath, "GDI+")

filePath := "D:\Images\MyPicture.png"
if oSize := GetImageSize(filePath)
   MsgBox, % oSize.W . "`n" oSize.H

GetImageSize(imageFilePath)  {
   if !hBitmap := LoadPicture(imageFilePath)  {
      MsgBox, Не удалось получить hBitmap
      Return
   }
   VarSetCapacity(BITMAP, size := 4*4 + A_PtrSize*2, 0)
   DllCall("GetObject", Ptr, hBitmap, Int, size, Ptr, &BITMAP)
   DllCall("DeleteObject", Ptr, hBitmap)
   Return { W: NumGet(BITMAP, 4, "UInt"), H: NumGet(BITMAP, 8, "UInt") }
}

pToken := Gdip_StartUp()
pBitmap := Gdip_CreateBitmapFromFile("c:\myImage.jpg")
Gdip_GetImageDimensions(pBitmap, w, h)
Gdip_DisposeImage(pBitmap)
Gdip_ShutDown(pToken)
MsgBox, % "Width: " w " Height: " h

filePath := "D:\Images\MyPicture.png"
oSize := GetImageSize(filePath)
MsgBox, % oSize.W . "`n" oSize.H
ExitApp

GetImageSize(imageFilePath)  {
   Gui, New
   Gui, Add, Pic,, % imageFilePath
   GuiControlGet, Pic, Pos, Static1
   Gui, Destroy
   Return { W: PicW, H: PicH }
}