I don't have a clue, I had the same thoughts as you.
Maybe this works and sets the image size to 128*128 for the output image file?
Didn't test it ....
GdipGetImageThumbnail,pImage,128,128,addr pThumbnail,NULL,NULL
GdipSaveImageToFile,pThumbnail,offset FilenameW,offset clsidEncoder,NULL