The choice of ultrasound frequency is a compromise between resolution and attenuation.
The resolution is the size of the smallest object that can be imaged. Since ultrasound is a wave (a sound wave), it will experience interference and diffraction effects like all waves, and as with all waves, the resolution will increase with decreasing wavelengths, so that smaller wavelengths will resolve objects more clearly. It may be thought then that use of the smallest wavelength and highest frequency is always desirable. This is not so. Attenuation increases with frequency, so if very high frequency ultrasound is used, it will all be absorbed by body tissue and none will be received at the probe.
In practice therefore a compromise is necessary between high resolution and low attenuation. In practice the best choice of frequency is often such that the part of the body being imaged is about 200 wavelengths of ultrasound away from the probe.