Per-Pixel Lighting

Ok, I have been reading alot about per-pixel lighting at the nvidia site as well in the Game Programming Gems1. I have considered adding it into my current engine in which i am working on right now. Well i understand most of the math behind it and understand how it works. How would i loud a gray-scale map or height map w/ opengl so i can get the normals on a per-pixel basis and what is this png file in all the demos?

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Here’s a link