Scrape your minds out of the gutter. Did it ever occur to anyone she’s his niece? 

Wake up. You are naive and possibly dull. Just look at that picture. That is not a scene of a platonic relationship.

Scrape your minds out of the gutter. Did it ever occur to anyone she’s his niece? 

According to the BBC in 2016, she was a married woman that he was in a confusing but technically platonic relationship

It also seems that she made a further declaration of her feelings for him while he was there, because the letter he wrote to her afterwards suggests he was struggling to make sense of the relationship in Christian terms. He tells her she is a gift from God, and goes on: "If I did not have this conviction, some moral certainty of Grace, and of acting in obedience to it, I would not dare act like this."

https://www.bbc.com/news/magazine-35552997#:~:text=Pope%20John%20Paul%20II%20was,rather%20sparse%2Dlooking%20living%20quarters.

If it's not your adultery it's none of your business.  

Agreed, as long as the parties involved aren't trying to get your approval (tactic or actual) of their actions.

Many Poles have a similar physiognomy, and John Paul had a classical Polish "look", high forehead, smallish eyes, slightly protruding lower lip.  My own son, who is half-Polish through his mother, had the characteristic forehead and lip in his youth, indeed, when he would pout when he got irritated, I'd tell him "put away that Polish passport" (i.e., the lip).  His features have matured more in late adolescence, such that it is not as noticeable, but when he was a baby and toddler, he was unmistakably Polish.

My point here, the fact that the child in the picture superficially resembles John Paul doesn't mean a thing.  There are kids who look like that boy all over Poland.  You could go in any grade-school classroom over there and find half a dozen boys who look just like that.

If it's not your adultery it's none of your business.  Trying to dig up a dead person's imagined sins from 40 years ago, AND scandalizing others by trying to get them involved in your waste of time is just placing black marks on your soul anyone's that participates with your trying to reveal the sins of another.
Read the 8th commandment.

Agree.

An innocuous photo proves zero

You think it's innocent because it wasn't photo proof of adultery in flagrante?  A photo showing a man associating with a women he shouldn't be associating with because of his state of life (married, priest, etc.) is already scandalous. It doesn't matter if they "did the deed" or not. Merely keeping private company with a woman who is not his wife is already a scandal. The fact that he was the Pope multiplies the scandal by 10000. 

That couple looks like they are on a date, period. Imagine if your husband were photographed with another woman, in the exact same poses, doing the same activities, with the same facial expressions, as seen in that picture. How would you feel?

Just looking at a woman is NOT the same as lusting after a woman.
That's not my opinion, that is Catholic moral theology.

It's black & white. Not confusing at all. Don't befuddle your own mind.

Matthew

God created women in a way that would attract men.
There are so many ways He did this, with their beauty and movements
He did this to encourage marriage to produce children.
Thus it is human for men to admire the looks of women.
I now read of adultery of the mind.
Can looking at a beautiful woman, created in a manner that attracts men,
just as God intended, now be classed as adultery.

The last thing a faithful Catholic wants is to commit adultery
when admiring a beautiful woman, yet it seems like a trap by doing so.
I can understand the sin of deliberately wanting to commit adultery,
but can this apply when seeing a woman you would like to marry for example.
Could someone please tell me how to do this natural thing
in a way that is not a sin.
The last thing a practicing Catholic man wants is to die and be accused of adultery of the mind

I could have written the above reasoning and question.
God created women in a manner that is attractive to men so that marriage would follow.
And that He achieved. I, now an old widower,
have feared that my attraction to such beauty could be judged as lust.
It is not intended by to be lust as a Catholic I know that to be a mortal sin.
Yet like my friend above I fear the 'trap' involved.
One cannot stop admiring feminine beauty.

Thank you Matthew for your answer.
Very comforting.