This scenario has been discussed extensively on the forum in some previous threads. This is probably the most recent that I know and I don’t think it’s been linked yet:
https://forum.inaturalist.org/t/can-i-use-photos-taken-by-my-grandparents-or-other-deceased-estate/59535
As noted above, observations are generally for the interaction between an observer and the organism.
Since it sounds like you weren’t present for the interaction in the wild, you could also upload these as a series of present day observations that are Not Wild.
I agree with others that there might be good non-iNat uses for these too!