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.