Great, I already have a PNG loader.
Now, as for specific question. Well, what pixels in a heightmap represent? Vertices of a triangle? Quads?
I was thinking that maybe I could draw one with triangle strips, considering each pixel as a vertex, and using the (X,Y) positions as indexes for indexed drawing, though I'm not so sure since I can't picture it on my head yet. Got set up the loader and a PNG file made up of random noise. I'm trying to code it right now.
I'm using LWJGL by the way.