I'm not exactly sure what you are describing in your first question. I mean, I understand what you are trying to do but I don't understand your description of what is happening. It sounds like instead of the picture appearing on the cube the picture is being drawn straight to the screen. In that case you are clearly A) not applying the image to the cube (eg. by copying the image to the texture buffer) and B) drawing the image to the screen using 2D commands like DrawImage.
As for the second question you can easily get vertex heights using commands like TerrainY (if you are using a Blitz terrain entity) or VertexY.
As for the second question you can easily get vertex heights using commands like TerrainY (if you are using a Blitz terrain entity) or VertexY.