Go get a haircut, and after the barber cuts too much off, tell him to put it back. Same thing with a pic, no matter jpeg or tif, you can't add back something that wasn't there. You only start with so many pixels, that's all you get to work with.
You can however, with Photoshop, enlarge it, then spend HOURS blending, smoothing, cloning, and blurring to get something that "looks" like the original. Best advice, convert it first to a tif, do your work, then convert back to a jpeg for the web.
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