Gave her a "I can't believe you are doing this" look but didn't say anything. She took another photo very quickly and then both girls left.
What would you have done?
I think that what you did was perfectly fine. It showed them that you were a bit uncomfortable with the extreme closeness.
They could have taken a step or two back, they could have lingered and ignored your "look" or they could have asked you if you would mind taking a picture of them up against the mural.
They did none of the above and
they left quickly. Which, in my mind, was the polite thing for them to do.
I don't find any rudeness in this crowded tourist situation from neither them nor you.