All of the PNG files can be found here These are all in PNG format already. Good programming! (a nice ZIP archive is also available Here) The ZIP archive contains all 17 of the Excel icons.
When you use the GetImageMso method, you end up with an IPicture interface to the object. The IPicture interface accesses the icon suitable for saving to a file in the original format - an .ICO, .WMF or a .BMP The PNG format is not supported. The following links explain why this is not directly possible:
http://msdn.microsoft.com/en-us/library/aa434604.aspx (msoGetImageMso method) http://msdn.microsoft.com/en-us/library/ms680761%28VS.85%29.aspx (IPicture Interface) http://msdn.microsoft.com/en-us/library/ms694504%28VS.85%29.aspx (Save As File method)
However, using a more complex approach will yield what you want: