Dim bmp as Bitmap=New Bitmap(100,100)
Dim g as Graphics=Graphics.FromImage(bmp)
g.DrawImage({Altes Bild},New Rectangle(0,0,100,100), New Rectangle(0,0,25,25), _
  PixelFormat....)
' oder einfach:
Dim bmp As Bitmap = New Bitmap({Altes Bild}, 100,100)