They've recently painted a huge mural of teenage activist Greta Thunberg on the side of one of the tall buildings in San Francisco

I can just see it now:
One of the city's destitute residents takes a dump on the sidewalk.  And, pulling up their pants, they turn around
...and, right above them, is a giant-size "Greta head" glaring down at them ever-so-sternly, as if to say
"I saw what you did just now!  That was pretty nasty, you know!"