A judge hearing the striker's sentence appeal at Belfast County Court reduced his original jail term from four months to three on Tuesday morning.
But Judge David McFarland declined to suspend the sentence, insisting a custodial term was appropriate, given the need to send out a strong "deterrent" to others.
Cliftonville FC player Donnelly, 23, of Ardilea Drive in Belfast, admitted the charge in November at Belfast Magistrates' Court.
He had been on bail pending the outcome of the appeal against his original sentence.
On Tuesday, family and friends wept in the public gallery of the court as Judge McFarland ordered Donnelly to be taken down to the cells ahead of being transported to prison.
In June 2016, Donnelly took a photograph of himself having sex with a 16-year-old girl who was wearing a Cliftonville shirt with his name and number on the back.
He shared the image with a friend and with a number of fellow players on the messaging service WhatsApp. It was later leaked on social media.
After the hearing Donnelly apologised to his victim.
In a statement issued by his solicitor, he said he accepted the court's decision.
He added: "It was accepted by the court today that since this incident occurred, three years ago, I had and continue to show genuine remorse for my action.
"I want to again apologise to the complainant in this case for the hurt that my action caused. This will never erase what happened, but I hope that today's outcome provides her with a degree of closure that allows her to move on with her life.
"I also want to thank my girlfriend, my family and my friends for the support they have shown me throughout this case.
"This is something I wish to move on from following my sentence, and I ask that my family are allowed to move on also, and are given privacy to do so."
The Irish Football Association (IFA) said it would now consider whether Donnelly had brought the game into disrepute.
"The safeguarding of children remains the highest priority of the association and our sympathy is with the victim and her family in this case," an IFA statement said.
Judge McFarland said the incident had had a deep and lasting impact on the victim.
He said Donnelly had treated the girl as a "sexual conquest or trophy" and had taken and shared the picture to "boast about his sexual exploits to friends".
The judge noted that the image would be online forever.
"It remains forever on the worldwide web, it's incapable of deletion," he said.
Donnelly showed no emotion as he was told he was going to jail. One supporter called out "we love you Jay" as he was led away by court guards.
Donnelly played one game for Cliftonville after he pleaded guilty to the charge last year, but was then dropped from the squad amid threats from feminist groups to protest outside the club's Solitude stadium in north Belfast if he was selected again.
He continued to be suspended from playing pending the outcome of the sentence appeal, although he remained on the club's books.
The judge said the picture content fell into the most serious category of indecent images of children.
"There is a need for deterrent sentences in cases such as these," he said. "This is clearly a case that requires a prison sentence."
He said aggravating factors in the case were the impact on the child and the fact Donnelly created the image himself.
In mitigation, the judge factored in Donnelly's previously clear record; his remorse; his age; the degree of verbal abuse he had suffered due to the case; and the fact he pleaded guilty.
"You created the image and were responsible for distributing it to a wider group and, indirectly through that group, on to the worldwide web," he told Donnelly.
The judge said the original sentencing judge had made only a "modest error" in the term handed down, as he reduced it by one month.
He again stressed that the impact of Donnelly's crime would endure.
"This was not momentary abuse that may fade from memory - this is continuing to impact on the young lady," he said.
The judge said while the victim was old enough to legally consent to sex, she was still viewed as a child under law in relation to the capture of indecent images.
He said the issue of whether or not she had agreed to the image being taken had not been resolved during the case.
But he said it was clear that she had not consented to the image being distributed by Donnelly.
Mr McFarland said he was not aware of the identities of all those in the WhatsApp group. He said attempts had also been made to delete some of the content.
"I understand some steps were taken to delete some of the messages and other information so the full picture is not available," he said.
Mr McFarland outlined the contents of the victim's impact statement in highlighting the seriousness of the offence.
He said since her name had been leaked on the internet, she had been subjected to verbal abuse.
The judge said incidents included children as young as nine shouting at her as she walked past a playpark; men outside a bookmaker's shop abusing her; men in a car stopping to abuse her in the street; and a number of Donnelly's friends turning up to her place of work to laugh at her.
"This has had a significant impact on this child, who is now of course an adult," he said.
The judge said Donnelly was a "talented footballer" but noted he had been assessed as having a low intellect, among the lowest 8% of the population.
He said while he did not have a psychiatric condition, a doctor had diagnosed him as having a "depressive disorder" as a result of the legal proceedings.
The judge noted the medic's view that a prison term would have a "significant impact" on Donnelly's mental well-being.
But he said his actions warranted a custodial sentence.
He said it was important a message went out that such conduct was "entirely wrong" and would receive "full punishment in the courts upon detection".