Image size when capturing a screenshot of another application changes if application not at 0,0

Question

I have an app which properly captures the image of an application window if it is in the upper-left corner of the primary screen.
But if it is not, the image size is not

Answer 1

Your code can be simplified in some details.
First of all, as already mentioned in the comments, your declaration of GetWindowRect() is not correct. You need to pass it a Window handle, usually in the form of an IntPtr structure, and a RECT structure.

Refer to the PInvoke website when you need to include a Windows API function call in your code. The experience of many programmers has forged :) those lines of code.

The Desktop size, here, is returned by SystemInformation.PrimaryMonitorSize.
You could also use Screen.PrimaryScreen.Bounds or SystemInformation.VirtualScreen.
Choose the one that best fits your plans.

Imports System.Diagnostics
Imports System.Drawing
Imports System.Drawing.Drawing2D
Imports System.Runtime.InteropServices

<DllImport("user32.dll")>
Private Shared Function GetWindowRect(ByVal hWnd As IntPtr, ByRef lpRect As RECT) As Boolean
End Function

<StructLayout(LayoutKind.Sequential)>
Public Structure RECT
    Public Left As Integer
    Public Top As Integer
    Public Right As Integer
    Public Bottom As Integer
End Structure

Private Sub BtnCapture_Click(sender As Object, e As EventArgs) Handles BtnCapture.Click
    Dim wRect As RECT = Nothing
    Dim WindowArea As Rectangle = Nothing

    Dim FindProcess As Process = Process.GetProcessesByName("calc").FirstOrDefault()
    If FindProcess IsNot Nothing AndAlso CInt(FindProcess.MainWindowHandle) > 0 Then
        If GetWindowRect(FindProcess.MainWindowHandle, wRect) Then
            WindowArea = Rectangle.FromLTRB(wRect.Left, wRect.Top, wRect.Right, wRect.Bottom)
        End If
    End If
    If WindowArea = Nothing Then WindowArea = New Rectangle(Point.Empty, SystemInformation.PrimaryMonitorSize)
    Using img As Image = New Bitmap(WindowArea.Width, WindowArea.Height, PixelFormat.Format32bppArgb)
        Using g As Graphics = Graphics.FromImage(img)
            g.SmoothingMode = SmoothingMode.HighQuality
            g.CopyFromScreen(WindowArea.Location, Point.Empty, WindowArea.Size, CopyPixelOperation.SourceCopy)
            img.Save("[The Image Path]", ImageFormat.Png)
            ScaleToClipboard(img, 65.0F) '65% of its original size or 
        End Using
    End Using
    '(...) Other processing
End Sub

Edit:
A method to save the original image to disk, reduce the source Image size to a specific size or to a fraction of it, then set the modified Image to the ClipBoard, ready to be pasted in some recepient.

ScaleToClipboard([Source Image], [Percent of Original] As Single)
ScaleToClipboard([Source Image], [Specific Size] As Size)

Example:
ScaleToClipboard([Source Image], 72.0F)
ScaleToClipboard([Source Image], New Size(200, 125))

Private Sub ScaleToClipboard(SourceImage As Image, SizeScale As Single)
    Dim NewSize As SizeF = New SizeF((SourceImage.Width \ 100) * SizeScale, (SourceImage.Height \ 100) * SizeScale)
    ScaleToClipboard(SourceImage, Size.Round(NewSize))
End Sub

Private Sub ScaleToClipboard(SourceImage As Image, SizeScale As Size)
    Using img As Image = New Bitmap(SourceImage, Size.Round(SizeScale))
        Using g As Graphics = Graphics.FromImage(img)
            g.SmoothingMode = SmoothingMode.HighQuality
            g.InterpolationMode = InterpolationMode.HighQualityBicubic
            g.DrawImage(SourceImage, New Rectangle(Point.Empty, SizeScale))
            Clipboard.SetImage(TryCast(img.Clone(), Image))
        End Using
    End Using
End Sub