>I wanna let the user choose an image file from my GetFile() function, then use the default image browser of his computer to open the image.
>What command/function should I use? Any help of reference will be greatly appreciated, thanks very much!

you can do this:

oWSH = CreateObject("wscript.shell")
oWSH.Run("D:\yourdir\yourfile.jpg")

assuming that Windows Scripting Host is installed. Alternatively, you can use Windows API. Check FAQ #8134.