820kHz and 2.5MHz (=3*820kHz) are eigenfrequencies of the mechanical system. You will be able to drive the transducer at any other frequency, but the damping will be greater by orders of magnitude, so it won't be usable.
Some ultrasound transducers can oscillate at both longitudinal and transverse mode. It is possible that that 820kHz and 2.5MHz are the natural frequencies for transverse and longitudinal mode while n*820kHz and n*2.5MHz are their harmonics. In this case, the transducer can be operated at both 820kHz and 2.5MHz with comparable amplitude but will produce different wave patterns. That does not neccessarily mean that any of the harmonics' damping is low enough to be usable.
Image from german wikipedia, public domain.
- The transducer in the top left image creates longitudinal waves by pushing and pulling a solid surface, liquid or gas it is pressed against.
- The left center one creates transverse waves in a solid fixed or pressed to its bottom (transverse waves cannot exist in liquids and gases).
- The lower left is the same as the one above it, but operated at the third harmonic.
- The three modes to the right are not used in ultrasound transducers.
There are also broadband transducers that do not operate in resonance. They can be used equaly well over a broad fequency range, but a resonance transducer operated at a correct frequency is much louder.