I managed to work it out. Here's how I did it if anyone is interested:
Dim inImage As MagickImage = New MagickImage(inImagePath)
Dim outImage As MagickImage = New MagickImage(Color.White, 1000, 1000)
Dim outGeom As MagickGeometry
Dim XStart as Integer = 0
Dim YStart as Integer = 0
Dim cellWidth as integer = 1000
Dim cellHeight as integer = 1000
outGeom= New MagickGeometry(New Rectangle(XStart, YStart, cellWidth, cellHeight))
outImage.CopyPixels(inImage, outGeom, 0, 0)
outImage.Write(outImagePath)