Is it true that we want the highest possible frequency to obtain the best image in the machine?

The answer is that you want to use the highest possible frequency so that you can observe the structures of interest in depth within the organization. The bold section is crucial - higher frequencies can provide better axial resolution, but at the cost of penetration depth. If the structure you want to image is 10cm away from the probe, then it is impossible to distinguish the first 1cm of tissue below the probe with high resolution! In this case, you must sacrifice some resolution (frequency) to achieve the necessary tissue penetration.

How to distinguish between mucus and pustules in cattle using B-ultrasound machine?

We hardly see Mucometra in the UK, and I have never personally examined animals with Mucometra. However, my understanding is that the mucus in the uterus is very watery, so it is usually silent during ultrasound examination, while in the case of pustule disease (which is more common in the UK), the substance in the uterus has a high degree of cellular and therefore echogenicity. The appearance is basically a homogeneous "granular" appearance with moderate echogenicity.