Prosecutor Gerrie Nel said Ms Steenkamp's head "exploded" when it was struck by one of four hollow-point bullets that the double-amputee runner fired through a closed toilet door in his home last year.
The photograph showed a side view of Ms Steenkamp's head, with a mass of blood and human tissue on the back and upper parts. Her eyes were closed.
"It's time that you look at it," Mr Nel said on the first day of cross-examination of Pistorius.
"I remember," Pistorius said, becoming distraught and turning away from where the photo was shown on a TV screen next to him. "I don't have to look at a picture. I was there."
Mr Nel had set the stage for a rigorous cross-examination by demanding that Pistorius openly say he killed his girlfriend, sharply challenging him when he said he made a "mistake".
The prosecutor showed a video, earlier shown on Sky News, of the celebrated Olympic athlete firing a gun at a watermelon and referring to its deadly power as a "zombie stopper".
Defence lawyer Barry Roux had earlier objected to the gun video being shown, saying it was inadmissible character evidence and amounted to a legal "ambush" of the defence. Judge Thokozile Masipa allowed the video to be shown.
Then, comparing the video of Pistorius shooting the powerful .50-calibre handgun at a watermelon, which explodes, Mr Nel said to him: "You know the same happened to Reeva's head? It exploded."
Pistorius, his voice rising and starting to sob, said he was at the scene when Ms Steenkamp died and knew of her terrible head injury, and said his hands had touched her brains when he claims he tried to help her.
Pistorius, 27, has said he shot the model by accident on February 14, 2013, mistaking her for an intruder. The prosecution alleges he killed her on the early morning of Valentine's Day by firing through a closed toilet stall door after an argument. He faces a possible prison term of 25 years to life if convicted of premeditated murder.
After the dramatic and aggressive start to his cross-examination, causing Pistorius to break down and the judge to call a recess, Mr Nel also started to poke holes in details of Pistorius's version of the events of the fatal night.
The champion runner conceded that his claim in a statement a year ago that he went out on to a balcony at his home before the shooting was incorrect. Pistorius said he went to the edge of the balcony but not outside.
And Mr Nel tried to dismantle the sympathetic image of Pistorius that the defence had sought to build up in three days of evidence. He opened by asking the athlete to explicitly acknowledge that he killed Ms Steenkamp.
"I made a mistake," Pistorius said.
"What was your mistake?" Mr Nel shot back.
Pistorius then said he "took Reeva's life".
"You killed her," Mr Nel said. "You shot and killed her," and he asked Pistorius to say it. Pistorius would not, saying merely: "I did."
Mr Nel tried to drive a wedge between the rosy former image of Pistorius and the ideals the runner has said he aspires to, and the prosecution depiction of the runner as a hothead with a gun obsession.
The prosecutor asked Pistorius if people looked up to him as a sporting hero, if he would not hide anything and if he lived by Christian principles.
"I'm here to tell the truth, I'm here to tell the truth as much as I can remember," Pistorius said. He also said: "I'm human. I have sins."
Earlier, Pistorius had kicked at and swung a bat at the bullet-marked toilet door, which had been placed in the Pretoria courtroom as evidence. It was a re-enactment of parts of the night when he killed Ms Steenkamp. He said he tried to kick the door down with his prosthetic legs and then bashed it with a cricket bat, an attempt to show he had tried to help Ms Steenkamp.
Pistorius also described what he said were the last moments of his girlfriend's life and how he dragged her, bleeding and "struggling to breathe", out of a toilet cubicle and downstairs to get help after shooting her in the head, arm and hip.
He said she died in his arms before paramedics arrived at his house.
"Reeva had died while I was holding her," Pistorius said, telling how he put his fingers in her mouth to try to help her breathe and put his hand on her hip to try to stop bleeding from one of several gunshot wounds.