Since most people probably don't own Designer, I thought I would point out that it is also possible to make 3D stereograms with an easy to use, inexpensive stand-alone application called Stereo 3D . You can cut and paste any of those into your paint program and save it as a file.
Seamless tiles are also available from a multitude of download sites on the web.
With respect to the grayscale image you want to give a 3D appearence to, the lighter grays will become the high-points in the resulting stereogram and the darker grays will become low points. But you shouldn't let this stop you from using simple, hand-drawn images to make stereograms for fun. With those caveats in mind, making the final image is trivial. The default depth setting works pretty well. Rendering is fast, even on my creaky old 7500.