She offered to send me photos and I completely froze

My girlfriend knows about this and is completely relaxed about it. Last week, out of nowhere, she said she could send me a photo sometime if I liked, in the same tone as offering to pick up milk.

And I did not know what to say. I think I said something like, yeah, maybe, which is a terrible answer to a genuinely kind offer. I have been mildly annoyed with myself ever since.

The bit I am stuck on is that I do not know what is reasonable to say yes to, or whether asking for anything specific makes it weird. How do people handle this without it turning into a demand, and is there anything sensible to agree about beforehand?