lol, it's normal to have pictures of us with our co-workers. I have pictures of me and our already married secretary inside a car while on a road trip with friends together with her husband so it doesn't mean anything.

I think the reason is simple, the guy likes the girl and he is showing his affection coz he's not aware that she has a Bf and sometimes some guys misunderstands the politeness that a women shows and thinks that the woman has a physical attraction towards him also.

Again if you trust your GF then don't worry, just tell your GF not to talk too much with your co-worker. so to avoid confrontation between you and your co-worker.

"Do not do unto others what you don't want others to do unto you(Wag kang gay-an, kung mang gagay-an ka, wag gay-on)"