Why were the Stoics named after a painted porch?

Because they met under one. Stoicism takes its name from the Stoa Poikile, the "painted porch": a colonnade on the north side of the Agora in Athens, decorated with mythic and historical battle scenes. Near the end of the fourth century BCE, Zeno of Citium and his followers gathered there to talk philosophy.

Zeno was not born a philosopher. He came from Citium, on Cyprus, and was a wealthy merchant before he turned to philosophy. On a voyage from Phoenicia to Piraeus he survived a shipwreck, went to Athens and visited a bookseller. There he read about Socrates and asked where men like him could be found. Just then the Cynic Crates of Thebes happened to walk past, and the bookseller pointed to him.

Crates had a way of teaching that Zeno never forgot. To cure Zeno's embarrassment, he handed him a pot of lentil soup to carry through the pottery district, then smashed the pot with his staff. As Zeno ran off with soup flowing down his legs, Crates called after him: "Why run away, my little Phoenician? Nothing terrible has befallen you." Like most of what we know about Zeno's life, the tale comes from a later biographer whose anecdotes are generally considered unreliable, so treat it as a legend that fits the man.

Around 300 BC Zeno began teaching in the stoa, and people started calling his school after the building. Unlike the Epicureans, he chose a public space. Stoic ethics centres on virtue as the highest good, with emotional self-control, calm problem-solving and rational judgment, all aiming at a lifelong flourishing the Greeks called eudaimonia.