Does her phone upload to the cloud? If so I would be changing that password and getting access.
1. Trust your gut. If it walks like a duck it's most likely a duck. Don't give it traction, get answers or you'll be having this conversation in divorce court a year or so from now, and you'll be devastated by the betrayal.
2. People that cheat hide their phones, and social media from their spouse. If she were being pursued and she wasn't interested or encouraging him, then she should have spoken to you about it.
3. She's already on a slippery slope. Time to decide what side of the isle she wants to stand on.
4. If this is workplace behavior it needs to be reported. They may both get fired, or you may find out where there's smoke there's fire.
5. Your married, you shouldn't have secrets, "special friends" or messages that aren't ok for your spouse to view. Same for photos. A respectful spouse would have shut that down in an instant and blocked him.
6. You already know something is going on, you're too afraid to confront her about your suspicions, because the reality of what it may be is too hard to cope with. Better to rip off the Band-Aid when it's possible to still heal the wound, than let it get gangrene later and need to be amputated. This may still be in it's infancy, and you may be able to salvage your marriage if you act proactively.
7. Choose yourself, and your well being over all else. Don't allow yourself to be disrespected, lied to and strung along. This is a 4 alarm blaze, either put out the fire and do preventative measures, or let the house burn and rebuild elsewhere. All are valid thoughts.
8. Walking this path hurts, but if you don't have trust you have nothing. Trust and communication are pinitol for a marriage. It will steal your sanity worrying, spying, chasing and wondering. Confront her with all the evidence. Copy the photos to something you can transfer them to. Print out what you've found. What's done in the dark must be brought to the light or you're never going to stop questioning where you stand with her.
I'm sorry you're faced with these suspicions. No one deserves to wonder where they stand in a marriage.