I post pics of my DSD's online (mostly myspace) but she does as well. In fact, she tells me if she taken/posted new pics of them on her myspace (she's on my friends list) so that I can use them as well.
If they're pics of the kids that I or DH have taken and are in our possession, I don't see a problem with posting them. We're their legal guardians also. Plus, she likes to post pics of them, so why would she have an issue with us doing it?
But then, we have an odd kind of friendly coexistence. I can see if one parent doesn't want pics posted for privacy reasons, and doesn't post pics of the child themselves online, then it would be an issue. That's understandable.
If the pics bother you because you don't want ANY pics of your child online, ever, then I would mention it.. or mention it to your ex. But if it's just because they're your kids and not hers so you think pic posting is your right... I would just deal. She's proud of them, and loves them. Plus it's hard for stepmoms to know the boundaries if they aren't set in stone to begin with. She probably didn't mean any harm in it.