Hi:

That sounds like a cross-modulation problem, rather than images. Images
would appear 910 kHz away.

Cross-modulation would occur in the rf stages or in the first mixer. To
determine if that is the problem, reduce the signal strength and the
cross-modulation will go away.

The SX-28 should not be cross-modulating under those conditions and should
be able to handle fairly strong signals. Often, the culprit is that someone
has modified the set for "improved sensitivity", in which case, they made
changes to make the S-meter read higher. Frequently, this involves changing
rf amplifier tubes from remote cutoff types to sharp cutoff types, or
changing resistors or bias voltages.

Check the circuit to make sure it is stock. Also, check the caps and
resistors in all of the circuits, especially in the rf, mixer and avc
circuits.

Also, realignment of the rf stages may help, especially if the tuned
circuits amplify the offending signal more than the desired one.

73, Colin K7FM