The photo was taken on a picnic when the former student, known in court only as AB, was 14 or 15, and had been circulating around her school.
Before he saw a picture of her in a "vulnerable position," she "went unnoticed" by Dawson.
"I really had very little to do with him," she told the NSW District Court on Tuesday.
"There was no need, he wasn't my teacher. I was ensconced in my social group and trying to navigate that normal 15-year-old life," she said.
Dawson, 74, is on trial accused of engaging in sexual activity with AB between July and December 1980 while she was aged 16 and his student on Sydney's northern beaches.
The carnal knowledge charge, which Dawson denies, is a historical offence replaced in 1986.
Prosecutors will have to prove any alleged sexual activity occurred while he was her teacher, public defender Claire Wasley said.
Crown prosecutor Emma Blizard said it is not in dispute that sexual activity took place between the two, who later married.
"The central issue will be one of timing and when events commenced," Ms Blizard said.
Dawson said he wanted AB in his class to get to know her, telling her that numerous times.
"Because it softened me," she said, giving evidence to a closed court on Tuesday.
"I felt special, because he'd said that."
He paid particular attention to her and she began confiding difficulties she was having at home, before he allegedly put his hand on her bare knee at a sports carnival.
"That was the first time he paid me that intimate - as I recall, as I see it - intimate attention," she said.
AB later started babysitting Dawson's children, almost every weekend, including times when Dawson and his wife were spending the night at home.
He began attending the licensed premises where AB and other students drank underage on Friday nights, telling her stories about "how fabulous he was" as "part of the grooming process".
"It just got out of control in terms of the amount of time I was spending with him," she told the court.
She was also brought along to exercise classes Dawson taught with his twin brother.
"When one twin was running the class the other was out the back with whichever student was their property at the time," AB said.
Ms Wasley reminded Judge Sarah Huggett she is not required to judge Dawson by contemporary legal or moral standards, and what she knows about his conviction for murder should have no bearing on the trial.
Sexual activity commenced between Dawson and AB while she was under 18 and a student but not while he was her teacher, Ms Wasley said.
Dawson was jailed in 2022 for murdering his wife Lynette in January 1982, with a judge finding he was motivated by a desire to pursue an unfettered relationship with a student.
Health issues in his old age mean Dawson will be given more frequent breaks during the judge-alone trial.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
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