If some of those originals came with the fine print stating they cannot be altered or reproduced, that's not good.
Thats the standard, default, automatically granted copyright under the Berne convention that every artistic creation, like a photograph, has, unless a more permissive licence like CC is explicitly expressed to cover the work. You cannot make derivative works of copyrighted works, without permission from the author, much less for profit, and much less without even crediting the original work. It is just wrong from every possible perspective.
Like you say, there are many images under Creative Commons licenses that she can use with no problem whatsoever. Also, you say it was evident to you that her works are not from scratch and that she was taking credit for the transformations, that is not the case, if you dont credit the original work, you are implicitly taking credit for the whole, just like if you write something and cite nothing. Moreover, if what she wants to take credit for is the transformations, then the audience needs to see the original image in the post, to see what exactly that transformation consists in, dont you think?
or how can you assess the transformation without seeing the original?
I'm aware. Assume copyright. That's the industry standard. Much of that exists to protect the work of the starving artist, so they don't have to lawyer up and fight corporate thieves.
You're right about providing sources. I can't argue this. Getting permission from the original creator is sometimes as easy as sending an email, if that's a requirement. She failed to follow standard practices.
This artform is welcome here but it does come with a lot of loose ends that need to be tied up before it can be published. It would be wrong to give some artists special treatment and let a few things slide. Fairness in a world that seems so unfair, right?