I think, without a back story, it should be totally fine to send photos like you describe.
But I can see circumstances where it might be less ok. It depends on his approach to other things in lots of ways. Mostly, the issue isn’t the photo sending. That’s just the shoot that you’re currently trying to deal with, not the huge rhizomatic root system that makes the weeds impossible to irradiate.
So a situation where he’s clearly pining after his ex and what could have been… that’s the issue that needs addressing.
A situation where he has double standards. You aren’t allowed to share any information (however basic) with your ex, but he’s swapping photos merrily with his ex, is indicative of a much bigger problem in which he feels he can control you while, utterly hypocritically, holding himself to entirely different standards.
And so on. Thing is, photo sharing wouldn’t be the only way in which the big problems were manifesting.
But, if it’s just him sending a photo of her riding a bike for the first time without any other issues, then it’s probably not an issue. Are there other problems that make you feel that he’s focusing on the ‘first family relationships’ at the expense of your shared children? Or is it just photos?