Risolto, ecco ho trovato questa funzione in giro per la rete per prendere il pezzo d'immagine che m'interessa:
codice:
    Private Function Crop(ByVal Source As Bitmap, ByVal x As Int32, ByVal y As Int32, ByVal               width As Int32, ByVal height As Int32) As Bitmap
    	Dim Cropped As New Bitmap(width, height)
    	Dim g As Graphics = Graphics.FromImage(Cropped)
    	g.CompositingQuality = Drawing2D.CompositingQuality.HighQuality
    	g.SmoothingMode = Drawing2D.SmoothingMode.HighQuality
    	g.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
    	Dim rect As New Rectangle(0, 0, width, height)
    	g.DrawImage(Source, rect, x, y, width, height, GraphicsUnit.Pixel)
    	Return Cropped
    End Function
Ciao e grazie per l'attenzione