Yes, the thresholds are set such that the object of interest lies between them. When it's color is distinct from that of the background, the histogram makes this easy and you can take threshold values before and after the object's "blob" in the histogram.
The third image, "shading.png", is more of an example that highlights how your thresholds can select a range of greys. There is nothing to segment here so set them however you want, just try a couple different values.
