Gerrie Nel said the model's skull 'exploded' when it was struck by one of four bullets that the double-amputee runner fired through a closed toilet door in his home last year.
The photograph showed a side view of the her 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 thundered indignantly on the first day of his cross-examination.
'I remember,' Pistorius said, becoming distraught and turning away from where the photo was shown on a TV screen next to him to audible gasps in the courtroom.
'I don't have to look at a picture. I was there.'
Pistorius, his voice rising and starting to sob, said his hands had touched her brain tissue when he claims he tried to help her after the shooting.
Mr Nel shouted: 'You made a mistake? You killed a person, that's what you did! You used to be a model to disabled athletes everywhere.
'You shot and killed her, won't you take responsibility for that?'
He asked Pistorius to say he 'shot and killed her', but the amputee runner refused, replying 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 wouldn't 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.'
Aimee Pistorius was overcome with emotion as she listened to her brother's testimony on Tuesday
After a brief adjournment, the court then viewed video footage previously broadcast on Sky News of Pistorius firing a handgun at a watermelon at a shooting range.
As the melon disintegrates, a male voice off-camera that sounds like Pistorius says: 'It's a lot softer than brains. But (bleep) it's like a zombie stopper.'
Pistorius admitted it was his voice, leading to Mr Nel to press him on his motives for wanting to see the water melon explode.
'You know that the same happened to Reeva's head. It exploded. I'm going to show you, the exact same effect (of) the bullet that went into her head,' he said.
As he spoke, Mr Nel projected a forensic photograph of Miss Steenkamp's head, with the side and back matted with blood and brain tissue, on the court monitors.
'Take responsibility for what you have done,' he told Pistorius, eliciting gasps from the packed public gallery.
Pistorius hid his head in his hands in the witness stand, rocking from side to side, sobbing and saying he took responsibility 'but I will not look'.
During an adjournment to allow the athlete to compose himself, Miss Steenkamp's mother June told reporters that she had agreed to it being shown because she wanted him to see it.
MailOnline has chosen not to show the picture.
After the dramatic start, prosecutor Mr Nel also started to poke holes in details of Pistorius's version of the events of the fatal night.
The champion runner said that his claim in a court document a year ago that he went out onto a balcony at his home before the shooting was incorrect.
Pistorius said today that he went to the edge of the balcony but not outside.
Mr Nel tried to pin down Pistorius on whether he meant to fire into the toilet cubicle door at a perceived intruder or whether his gun discharged accidentally.
Pistorius said he didn't intend to shoot 'anyone' and that he fired 'before thinking' because he thought his life was in danger, prompting Nel to accuse him of weighing the legal implications of the question before answering.
Pistorius could still be convicted of murder on his account that he thought he shot at an intruder - unless he can convince the court that he shot on reflex and not intentionally.
Pistorius told the prosecutor that he was trying to be careful with his answers because the stakes were high.
Mr Nel snapped back: 'Reeva doesn't have a life any more because of what you did. Please answer and don't think of implications to you.'
Earlier, Pistorius described how Reeva Steenkamp died in his arms as he made frantic attempts to resuscitate her and stop the bleeding after gunning her down.
The double-amputee was giving evidence for a third day, returning to the witness box after wailing yesterday while describing the moment he realised he had fatally shot his girlfriend by mistake.
He also denied any intention to kill girlfriend Miss Steenkamp or the intruder he believed to be behind the door.
In early evidence today, Pistorius mostly kept his composure, though his voice quavered while describing what he said were his desperate attempts to save the 29-year-old model.
He said: 'I knelt down over Reeva.
'She was sitting with her weight on top of the toilet bowl. I checked to see if she was breathing and she wasn't.
'I pulled her weight on to me and I sat there crying for some time.
'I had her head on my left shoulder and I could feel her blood was running down on me. I could see that her arm was broken.
He said he could barely pick her up, so placed her 'softly' on a bathmat while he went to open the bedroom door and the front door.
He then went back to the bathroom and carried her downstairs.
In between sobbing and long pauses, he said: 'I couldn't pick her up.
'I was struggling to pick her up. I was scares that I had hurt her more. I put her arm onto her body.'
At that point, he came across neighbour Johan Stander and his daughter.
'Before I put her down, I said "we need to get her to the hospital, we need to get her to the hospital".
'They said the ambulance is on its way. Then I just sat with her and waited for the ambulance to arrive.
'I had my fingers in her mouth to help to breathe. I had my hand on her hip to try to stop the bleeding'.
Pistorius, who has not consented to being shown while giving evidence, said paramedics arrived a short time later.
'I stood back when they arrived and stood a couple of metres away from them,' he added.
