I'm a little confused by your question. If you're talking wallpaper and an image you are either capturing with a camera or creating somehow, wouldn't it be a matter of aligning whatever you want in that space to achieve the effect you're looking for? I guess I'm stumped on the pixel thing. Since most manufacturers describe their screen resolution in pixels per square inch, for example 1920 x 1080, you could approximate the number of pixels by measuring the physical screen in that area and using your devices pixel density to figure it out. I don't think you could do that from a screenshot as the scale would change if you attempted to print it out or display it since the pixel density is based on the physical screen dimensions.
Best...