Cool: that it works in a decent way, and uses a neat trick.

Uncool: You can't export the image into a 3D file, making it 99% less useful that it would be otherwise.

Suggestion: export it as VRML, it's trivial format that you can generate starting from your points. Use this format (from my own code, so use it as you wish):

    fprintf(fp,
    "#VRML V2.0 utf8\n"
    "Shape {\n"
    "    appearance Appearance {\n"
    "        material Material {\n"
    "            diffuseColor .5 .5 .5\n"
    "        }\n"
    "    }\n"
    "    geometry ElevationGrid {\n"
    "        xDimension  %d\n"
    "        zDimension  %d\n"
    "        xSpacing 0.01\n"
    "        zSpacing 0.01\n"
    "        solid FALSE\n"
    "        creaseAngle 6.28\n"
    "        height [\n", hgt->width, hgt->height
    );

    for (y = 0; y < hgt->height ; y++) {
        for (x = 0; x < hgt->width; x++) {
            height = getheight(x,y);
            h *= YOUR_3D_MULT_FACTOR;
            fprintf(fp, "%f", h);
            if (y != hgt->height-1 && x != hgt->width-1) fprintf(fp,",");
            if (x == hgt->width) fprintf(fp,"\n");
        }
    }

        fprintf(fp,
    "        ]\n"
    "    }\n"
    "}\n"