The photo is evidence of the observation made by the iNatter (much prefer that term to “user”!). It stands to reason that a photo copied from a book (or elsewhere on the net) is not evidence of the actual individual observed, but a photo taken by someone else that was there would count as evidence that it was observed there. As long as it is a photo of the actual organism seen, and assuming the “owner” of the photo gives permission to use it, then it would be appropriate evidence!
If there were 10 people all gathered around the same thing, all taking photos and putting up observations of the same organism, then that is definitely fulfilling the goal of iNat! People connecting and sharing a passion for the natural world, encouraging and supporting one another in that pursuit… I would happily ID those 10 observations… if it is obvious they are the same individual, then it is even easier to ID the subsequent observations, especially if they all upload at around the same time as well!
It would be courteous to give details in the case of a shared photo, and perhaps even link to other peoples observations of the same thing, but not all iNatters are that tech savvy or tech passionate to do so…