Each image is scanned with 240 dpi while Windows default to 96 dpi. That means, the image is scaled down, so I simply get the least dpi and set it as dpi for the combined image. It works for now, though I can see the potential problem if somehow the horizontal and vertical dpi are different.
Dim image1 As Bitmap = ...(code to get image) --> has 240 dpi Dim image2 As Bitmap = ...(code to get image) --> has 240 dpi Dim combinedImage As New Bitmap(...) --> default to 96 dpi Dim horizontalResolution As Single = Math.Min(image1.HorizontalResolution, image2.HorizontalResolution) Dim verticalResolution As Single = Math.Min(image1.VerticalResolution, image2.VerticalResolution) combinedImage.SetResolution(horizontalResolution, verticalResolution)