How did people in the wizarding world know what Harry looked like?

Question

In Harry Potter, Harry had no idea about the wizarding world until Hagrid came to get him. Since he was sent to the Dursleys as a baby, how do the other witches and wizards know what he looks like when he shows up at Hogwarts, and the Leaky Cauldron? It couldn't have been his scar, because in the Leaky Cauldron, everything was dark, and Harry had his bangs over the scar. In fact, Harry always had to lift up his bangs to let people see it.

"He pointed at Harry's forehead. Harry pulled back his bangs to show the lightning scar."

^{(Harry Potter and the Philosopher's Stone -- chapter 6, page 99)}

So how did the wizarding world know what Harry looked like before he knew about wizards?

Answer 1

Presumably, everyone knew about the scar, as the twins asked about it in Harry Potter and the Sorcerer's Stone:

“What’s that?” said one of the twins suddenly, pointing at Harry’s lightning scar.
“Blimey,” said the other twin. “Are you —?”
“He is,” said the first twin. “Aren’t you?” he added to Harry.
“What?” said Harry.
“Harry Potter.” chorused the twins.
“Oh, him,” said Harry. “I mean, yes, I am.”

So, really, the Weasly twins didn't know he was Harry until they caught sight of his scar. This is his defining feature and what causes people to recognize him!

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Also, as mentioned by @Gaultehria's post, James Potter was also fairly well-known, and we know that he looked like his father (except for the eyes!).

Answer 2

Harry looks like his famous father

Although Harry himself is the most famous member of his family, his father James and mother Lily are famous in their own right due to their sensational murder case, and their images are widely known from media coverage and tributes such as their well-visited memorial statue in Godric's Hollow.

Throughout the series, people remark that Harry looks very much like his father. Many of these people had the benefit of meeting James, but it's possible that others could recognize him secondhand. Additionally, Lily's eyes are distinctive enough to be notable. People tell Harry that he looks like his father except that he has his mother's eyes; they don't phrase it as "You look like your father, except that your eyes are green."

While wizarding media may have only had access to pictures of Harry as an infant and a toddler, Harry's strong resemblance to his parents — especially to his father — would be enough for some people to identify Harry, and to prompt them to look for the scar on his forehead (assuming that detail was known or rumoured).