I am developing a C# application and I pass along 2 check images to a function. Can you tell by the file I attached that contains snapshots what the error is?

I am developing a C# application and I pass along 2 check images to the function that follows.
The 1st check image is the Front of a Check
The 2nd check image is the Back of a Check
In the attached file you can see snapshots of the error I received:
Do you see any information in the attachments that would indicate why the error occurred?

ArgumentException was unhandled of type 'System.ArgumentException' occurred in System.Drawing.dll

Additional information: Parameter is not valid.

public static Bitmap MergeTwoImages(Image firstImage, Image secondImage)
        {
            if (firstImage == null)
            {
                throw new ArgumentNullException("firstImage");
            }

            if (secondImage == null)
            {
                throw new ArgumentNullException("secondImage");
            }

            int outputImageWidth = firstImage.Width > secondImage.Width ? firstImage.Width : secondImage.Width;

            int outputImageHeight = firstImage.Height + secondImage.Height + 1;

            Bitmap outputImage = new Bitmap(outputImageWidth, outputImageHeight, System.Drawing.Imaging.PixelFormat.Format32bppArgb);

            using (Graphics graphics = Graphics.FromImage(outputImage))
            {
                graphics.DrawImage(firstImage, new Rectangle(new Point(), firstImage.Size),
                    new Rectangle(new Point(), firstImage.Size), GraphicsUnit.Pixel);
                graphics.DrawImage(secondImage, new Rectangle(new Point(0, firstImage.Height + 1), secondImage.Size),
                    new Rectangle(new Point(), secondImage.Size), GraphicsUnit.Pixel);
            }

            return outputImage;
        }
Image-files.msg