I would do what you are comfortable with. Sending pictures is sharing with YOU experiences of the kids. It's kind of like a gift to you from her. Do you want her to stop? What is your goal by not reciprocating? She seems open to text/pic communication. That's a door that I would not slam shut unless you wanted.
BTW, I don't really think it's important they are going anywhere for the first time. Yeah, it makes it special, for you. But, you need to always remember, it's not what you do, it's just the time doing. It does not have to break the bank. Silly example, about a month after I was separated got a place and 5D stayed with me for a few days. I built a fire in the backyard sat on blankets, read stories, hot chocolate, roasted marshmallows under the stars, moon, etc...Spent an hour maybe, maybe 2 hours doing that 5 months ago. She still talks about it. Something simple. It's just the time. It doesn't have to be perfect. And that makes just a good a picture. That's all they want is your time to love them.