grazie mille!
ho trovato anche questa:
codice:
Public Shared Sub MakeImageGrayscale(ByVal bmp As Bitmap)
	Dim cMatrix As New ColorMatrix(New Single()() _
	{New Single() {0.299, 0.299, 0.299, 0, 0}, _
	New Single() {0.587, 0.587, 0.587, 0, 0}, _
	New Single() {0.114, 0.114, 0.114, 0, 0}, _
	New Single() {0, 0, 0, 1, 0}, _
	New Single() {0, 0, 0, 0, 1}})
	Dim imageAttrib As New ImageAttributes
	imageAttrib.SetColorMatrix(cMatrix)
	Dim gr As Graphics = Graphics.FromImage(bmp)
	'Apply the grayscale image attribute
	gr.DrawImage(bmp, New Rectangle(0, 0, bmp.Width, bmp.Height), _
	0, 0, bmp.Width, bmp.Height, GraphicsUnit.Pixel, imageAttrib)
	gr.Dispose()
End Sub