Image to bank problems

Const screenwidth=640
Const screenheight=480
Global img%

Function CopyBankToBuffer(bank3,buffer)
; lock the image for byte transfer
   LockBuffer buffer
; test locked imagebuffer is 16 bits per pixel
   If LockedFormat(buffer)<>1
      UnlockBuffer buffer
      Notify("this test is 16 bit display mode only")
      End
   EndIf
; cache buffer variables   
   imagebank=LockedPixels(buffer)
   pitch=LockedPitch(buffer)
; copy bank to image line by line   
   For y=1 To screenheight
      CopyBank bank3,srcoff,imagebank,destoff,screenwidth*2
      srcoff=srcoff+screenwidth*2
      destoff=destoff+pitch
   Next
   UnlockBuffer buffer
End Function

Function image2bank(bank3)
DrawImage img%,0,0
   LockBuffer BackBuffer()
   For y=0 To screenheight-1
      For x=0 To screenwidth-1
         arg=ReadPixelFast (x,y,BackBuffer())

         ; Extract 1 byte Red, Green, Blue values, and divide by 16 to convert to
         ; a value between 0 and F - blue does not get divided and stays original
         argr=(arg And $ff0000) Shr 20
         argg=(arg And $ff00) Shr 12
         argb=(arg And $ff) Shr 4

         ; reconstruct the color value in 16 bit format
         ; ??? is the Blue part correct ???
         arg=(argr Shl 12)+(argg Shl 8)+argb
         PokeShort bank3,o,arg
         o=o+2
      Next
   Next
   UnlockBuffer BackBuffer()
End Function

Graphics screenwidth,screenheight,16,2
SetBuffer BackBuffer()

bank2=CreateBank(screenwidth*screenheight*2)

img%=LoadImage("x:\monkey5.jpg")
SetBuffer BackBuffer()
DrawBlock img%,0,0
image2bank(bank2)
Flip
WaitKey()

copybanktobuffer(bank2,BackBuffer())
Flip
WaitKey()
End