You misunderstand his question, possibly because he mixes up his vocabulary in his post by referring to the "scale of the texture" when the relevant information is dimensions and not scale. He was not asking about the texture on a terrain or about the rendered image of the terrain, he was asking about the heightmap image used to generate the terrain. That said, I have no idea. I doubt it (that is, I doubt each vertex corresponds to a pixel) but I'm not sure since I've never really used Blitz terrain. Logically there can only be a 1 to 1 correlation if the number of vertices is the same as the number of pixels in the heightmap image, and you can set the number of polygons to anything you want with the TerrainDetail command. It may be however that by default, before you use TerrainDetail, terrains are loaded with a number of vertices equal to the number of pixels in the image; I'm just don't know.
As for the second part of the question, whether one can determine the heightmap pixel based on knowing information like the vertex position and the heightmap image dimensions, well sure. A terrain is just a regular grid of vertices on the X and Z axes; the shape of a terrain is determined by deflections of the vertices on the Y axis. Thus you can use the X and Z coordinates of a vertex to find the corresponding position on the heightmap image.
As for the second part of the question, whether one can determine the heightmap pixel based on knowing information like the vertex position and the heightmap image dimensions, well sure. A terrain is just a regular grid of vertices on the X and Z axes; the shape of a terrain is determined by deflections of the vertices on the Y axis. Thus you can use the X and Z coordinates of a vertex to find the corresponding position on the heightmap image.