Putting aside whether you should or shouldn't be doing this - it's your choice. I know I would be wanting answers, but brace yourself for revelations and denials.
If you decide to investigate the photos:
Are they in work/business clothes, which he usually only wears for work, in a business environment? Then it's likely to be a colleague. The company website may have headshots of employees.
Has he been away recently for work? Are they in surroundings that fit where he was? Then he may have met someone there, or again, a colleague. The company website & socials may have pics from the same event.
Does he have hobbies, do sports? Are they in golf, walking, amateur theatre costume etc gear? Then this is someone he's met through that thing. Checkout their website & socials.
Say you'd love to do more interests with him and go along to the next one. Watch his face.
Do you know his colleagues, other people who go to the hobby or sport? You could copy the photos, take him out of them and enquire with them if they recognise her, as you have an important message for her.
Then you can see if she has social media, check her pictures, see if he's there. Be very careful what you say to her, but let her know there are a wife, young children and a home at stake. She may or may not know.
Or ... you could simply say to him 'I noticed a lot of pictures of a particular woman on your gallery and I wondered who she is.' Again, watch his face.
Then be aware you may not really know your husband.
All the best.