Cole Allen wore black trousers, a black shirt and a red tie as he snapped the image in his room at the Washington Hilton, where Trump and hundreds of journalists were meeting for a gala Saturday night, authorities said.
Allen, 31, of Torrance, California, was captured when he tried to race past security barricades near the hotel's ballroom, prompting an exchange of gunfire with Secret Service agents tasked with safeguarding the event, they say.
New details emerged in a court filing made by prosecutors who want Allen to remain in custody.
The US government said Allen repeatedly made online checks to keep track of Trump's status that night, including live coverage of the president exiting his vehicle at the Hilton hotel.
Investigators said preset emails with an "Apology and Explanation" attachment were sent at approximately 8.30pm.
"He intended to kill and fired his shotgun while trying to breach security and attack his target. Put simply, the defendant poses an uncommonly serious danger to the community if released pending trial. The defendant's lack of criminal history and other personal circumstances do not alter this conclusion," Assistant US Attorney Charles Jones wrote.
Trump was uninjured.
A Secret Service officer wearing a bullet-resistant vest was shot in the vest and survived.
Allen appeared in court on Monday and was charged with the attempted assassination of the president as federal authorities suggested an attack that disrupted one of Washington DC's glitziest events had been planned for at least several weeks.
He did not speak at length during the quick court appearance, as is customary.
One of his lawyers, Tezira Abe, noted he had no criminal record and said he "is presumed innocent at this time".
Trump, a Republican, was rushed off the stage by his security team on Saturday night and appeared at the White House two hours later, still in his tuxedo.
"When you're impactful, they go after you. When you're not impactful, they leave you alone," Trump said.
"They seem to think he was a lone wolf."
An FBI affidavit filed in the case on Monday revealed other details about the planning behind the assault, with authorities alleging that Allen on April 6 reserved a room for himself at the Washington DC hotel where the event would be held weeks later under its typical tight security.
He travelled by train cross-country from California last week, checking himself into the Washington Hilton one day before the dinner with a room reserved for the weekend.
