ajohn

The most obvious difference between male and female is size. Females are obviously larger than males. This can be very obvious when they are seen together, but I think would be difficult from a single photo of one individual.

ajohn

It looks like the immature is male and the adult female (although the apparent size difference may be partly due to the different angle). The size difference should be apparent even when the birds are immature (birds grow very little after they fledge).
I think that the adult Imperial that I have been seeing regularly at Mai Po this winter is female - it often seems huge!