That's a very good question. I'm pretty much going to leave it to others to answer.

However, I would say that my intuition tells me it is enough........

But, you are just getting the rights for it in the game. Say someone wants to make a plush animal or toy based on that graphic/character if your game becomes popular. There would probably be some serious negotiating to be done between you and the owner.

Edit: I see it is not a fictional character but people. Be careful. They might decide later they don't want their picture in your game.

In addition to getting the permission it writing, there is also some legal hair-splitting about all the people you need permission from.

You need written permission from the photographer, because they hold the copyright, moral rights, and assorted other IP rights. Your lawyer can help draft this. For single-shot photos the cost usually depends on the intended usage and size of distribution; anywhere from $150 to $1000 could be fair negotiated prices for a single use in a small-distribution game, but add a zero if it is used for promotional and advertising material. As an example of one of many photograph-industry pricing calculators, clicky. Obviously there are brand-name photographers, but for a generic image those prices are a good negotiation point.

Also depending on locations and uses you might need written permission from the people in the photo. If you are using the photo for advertising purposes you need permission no matter where on the globe you live. If you are using it for inside the game or other non-advertising uses, the need for permission from the photo model depends on your location on the globe.

Finally, there are usually more things in the photograph than the people. Are they sitting in a chair? Are they inside a building? Is there furniture? Is there clothing? Cars in the background? A candy wrapper or beverage with a logo? Designs for all of these are owned by a person or a business, and when you enter the realm of commercial use you may or may not need permissions for all of them, depending on how you use the images.

The safest answer is to check with a lawyer, and then get written permission from everybody your lawyer says you need permission from. You will need to specify to your lawyer and in the permission form what media will be used, how it will be used, and roughly how many instances will be used.

You need written permission from the photographer

I was going to say that. I've seen a case recently where someone was using a person's picture. There was a lawsuit, but ultimately, the user was able to exploit the picture because the person depicted was not the photographer (ultimately chose to blur the face regardless). That being said, the photographer came thereafter in the mix and that didn't go so well. You need written consent from the photographer more than you need written consent of the person depicted, but ideally, you need both.