Another witness said Graner was "laughing and having a good time" while making naked prisoners pose.
Spc. Matthew Wisdom, the first witness in Graner's prisoner abuse court-martial, said Graner was among a number of guards who roughed up detainees on Nov. 7, 2003. Graner is the first soldier to be tried in the case, and prosecutors say he was the ringleader of the abuse.
Testimony got under way Monday after opening statements.
Wisdom described a prominent photo from Abu Ghraib that showed the muscular Graner holding a detainee as if he were about to strike him in the face.
The witness said Graner cocked his arm while the picture was taken, and then he punched the detainee.
Asked how hard Graner hit the prisoner, Wisdom said, "If I was that detainee, I know that it would be very painful."
Wisdom said he was urged to participate in abuse, but that he instead reported it to his immediate superior.
"I was very upset," he said. "It made me kind of sick, almost. It didn't seem right."
Graner, a 36-year-old former prison guard from Uniontown, Pa., is charged with conspiracy to maltreat Iraqi detainees, assault, dereliction of duty and committing indecent acts.
The defense contends that Graner was told by higher-ranking soldiers and intelligence agents to rough up the detainees prior to interrogation, and that he had no choice but to obey despite personal misgivings.
An all-male jury of four Army officers and six senior enlisted men will decide his fate in what is expected to be a weeklong trial. If convicted on all counts, he faces up to 17½ years in a military prison.
Under military law, a conviction requires guilty votes by seven of the 10 jurors, all of whom have served in either Iraq or Afghanistan.
Wisdom testified that he did not see Graner when a naked Iraqi was allegedly forced to masturbate. That incident is the basis of one of the maltreatment charges against Graner.
But another member of the 372nd Military Police Company (search) said Graner was not only there, but that the defendant photographed a simulated oral sex scene.
Pvt. Jeremy Sivits, who in May pleaded guilty to taking part in abuse, said Graner was in charge of stacking naked prisoners into a human pyramid with which he later posed for pictures.
"He was trying to get the job done, but he was also laughing and having a good time," said Sivits, who told the court that his testimony against Graner was part of his plea deal.
Sivits received the maximum sentence of one year in prison, a reduction in rank and a bad conduct discharge.
Two other members of Graner's unit who have made plea deals, Ivan Frederick and Megan Ambuhl, are also scheduled to testify against him.
Sgt. Joseph Darby, who first reported the alleged abuse, also is scheduled to appear. As many as three Iraqi detainees may testify via videotaped deposition.
Three more soldiers from the 372nd also are awaiting trial at Fort Hood. One is Lynndie England (search), who in October gave birth to a child who Army prosecutors say was the result of a relationship with Graner.