According to today’s judgement in the High Court, the case is understood to be the first of its kind, where a UK court has considered whether online publication by a national newspaper is in breach of the 1981 act.
The newspapers were taken to court by the Attorney General. In February it was argued on his behalf that publication of the pictures online had created a substantial risk that the trial "could have been seriously impeded or prejudiced".
The newspapers had argued that the risk of prejudice was "insubstantial", partly because the judge at the murder trial had specifically told the jury not to look on the internet for information.
It was also argued that the images published still showing the gun were done so "by mistake" and for a limited amount of time, for about five hours on MailOnline and around 19 hours on the Sun Online.
Judgement was reserved until today, when Lord Justice Moses and Mr Justice Owen ruled that not only was there a substantial risk that a juror might read the news report but that the image was prejudicial.
The defendants' submission that the illustration of the accused "was so remote from the factual issues in the case that the Attorney cannot prove that there was a substantial risk that a juror would be adversely influenced by it", was rejected.
"Visual images are designed for impact; that is why any editor would be keen to use them to add to the impact of the news story," Lord Justice Moses added.
"...It was prejudicial in a manner directly relevant to the issues in the case."
In conclusion he said that "notwithstanding that publication of the image of the accused with a pistol was a mistake", the defendants were guilty of contempt under the strict liability rule.
"The criminal courts have been troubled by the dangers to the integrity and fairness of a criminal trial, where juries can obtain such easy access to the internet and to other forms of instant communication. Once information is published on the internet, it is difficult if not impossible completely to remove it.
"... Whilst in this case we have not been satisfied that a juror could have been told about the photograph by someone who had obtained access to the article through 'Twitter' or by registration to receive an "RSS feed", the ability to obtain news by such means must be acknowledged.
"... The courts, while trusting a jury to obey a prohibition on consulting the internet, have been concerned to meet the problem. This case demonstrates the need to recognise that instant news requires instant and effective protection for the integrity of a criminal trial.
"We conclude that the nature of the photograph created a substantial risk of prejudicing any juror who saw that photograph against the defendant".
The court has now invited submissions concerning penalty and costs.
Neither the Daily Mail or the Sun has responded to a request for comment.