Based on numerous replies, I begin to suspect you are not being very clear about what you hope to accomplish. I'll do my best to guess. That said, it would be easier, if you simply provided more detail in the question.
To view a file with an image viewer, you need to actually have a file. The file should be on-disk, not in memory. Because you specified "C:\testImage.jpg" in your question, I naturally assumed that you had already previously saved your image to disk. Your later responses, seem to indicate this may not be the case.
So, your first step, if the image is not already on disk, is to save it to disk. You don't mention what type of application (WinForm, WPF, WebForm, etc.).
For the purposes of this guess, I'll explain for WinForm. The way to save an JPEG image, from a WinForm picture box, to disk would be as follows:
string myPictures = Environment.GetFolderPath(Environment.SpecialFolder.MyPictures);
string myPath = System.IO.Path.Combine(myPictures, "testImage.jpg");
pictureBox1.Image.Save(myPath, System.Drawing.Imaging.ImageFormat.Jpeg);
To display that image, using some image viewer...
System.Diagnostics.Process.Start(myPath);
System.Diagnostics.Process.Start(myImageViewer, myPath);