Yeah, if she asks you "why?" or deflects it at all, then you're getting into what I talked about regarding resistance to compliance. You're just asking her for a pic, which is pretty common I find when I'm texting a new girl. She'll ask me for pics usually too.
It's a pretty simple request and you're not asking for a ton of compliance there, so if there's any questioning or resistance to that, then she's probably not very interested or else she's just testing you.
But yeah, it also could be just that she is oblivious to the context, so you could just give her a non-answer like "just cuz" or if you feel like investing a bit more to get her to bite, then you could send a slightly longer message (and to add context if you really think she just didn't get what you were asking for), like "Because I just thought it'd be nice to have a pic(s) of ya :)" or something along those lines.