Changing the area of a pixel search - vb.net

Question

I have been working on an app to search for 3 pixels in a certain area then play a sound. It currently works however I wish to reduce the zone of which it searches from the whole screen to a rectangular area on my screen, this is to save memory usage of the app, here's the code that works
Private Sub dcdTimer_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles dcdTimer.Tick
Dim screensize As Size = New Size(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
Dim screenshot As New Bitmap(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
Dim ge As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(screenshot)
Dim pointx As Integer = 1
Dim pointy As Integer = 1
ge.CopyFromScreen(New Point(0, 0), New Point(0, 0), screensize)
looking = True
Try
While looking = True
Dim atpoint As Color = screenshot.GetPixel(pointx, pointy)
Dim reckcol1 As Color = Color.FromArgb(a, r, g, b) 'first pixel colour
Dim reckcol2 As Color = Color.FromArgb(a1, r1, g1, b1) 'second pixel colour
Dim reckcol3 As Color = Color.FromArgb(a2, r2, g2, b2) 'third pixel colour
If atpoint = reckcol1 Then 'Matches 3 pixels
Dim testPoint As Color = screenshot.GetPixel(pointx, pointy + 10)
If testPoint = reckcol2 Then
Dim testPoint2 As Color = screenshot.GetPixel(pointx + 10, pointy + 10)
If testPoint2 = reckcol3 Then
My.Computer.Audio.Play(sounddir + "Found.wav")
looking = False
Sleep(15000)
pointx = 0
pointy = 0
End If
End If
End If
pointy = pointy + 1
If pointy = My.Computer.Screen.Bounds.Height Then
pointy = 0
pointx = pointx + 1
End If
End While
Catch ex As Exception
End Try
End Sub
I tried to use this code as suggested:
Dim pointofY As New Point(500, 400)
Dim screensize As New Size(400, 200)
Dim screenshot As New Rectangle(pointofY, screensize)
Dim ge As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(screenshot)
However, using a rectangle does not allow me to use getpixel or use ge.copyfromscreen