For deflecting photos waved in my face, I've answered with "Uh-huh" and went on with my work/moved away/shifted conversation elsewhere. I've also responded with, "Yup, s/he's still a baby all right". I have one co-worker who lately is on a Grandma Pride rampage because her grandson just hit middle school and is growing up fast, looking much older, etc. When she showed me a photo of him, all I did was nod. She was elated. On the inside I was laughing that she'd get that much of a charge out of a NOD, but on the outside I went back to eating my lunch.
If it's a situation where politeness and tact are required but I just really don't know/don't care about their kid...or it looks like every other damn baby out there...I will fall back on neutral descriptors that parents take as a compliment to either them, their parenting, or their kid. Stuff like "S/he's so small", "I can tell s/he's grown", or "looks like you/your spouse". None of those are actual compliments. Babies are small, they grow as they age, and if they're not adopted they will often resemble one or both of their parents. But parents eat that shit up. If I really can't find anything neutral to say about the kid, I will compliment the cute little overalls it's wearing or comment on the weather in the picture.
My favorite neutral comment, though, is "Wow". It's great because I can deliver it in a polite, straight tone but mean absolutely anything by it. And in my head, I can follow with what I'm really thinking, like "...that's what your life consists of now?" "...you're easily impressed." "...I am so glad birth control is a thing" and so on.