The alleged victim was under 16 when it's claimed Fr Anthony McSweeney attacked him at Grafton Close Children's Home in Hounslow in the 80s.
The man, now in his 40s, who can't be named for legal reasons, claimed Fr McSweeney abused him before producing a camera and taking indecent images.
Jurors were told: "He stopped and picked up an old camera - the kind that put photographs out straight away...and he took a couple of photographs."
The alleged victim added: "When I called an end to it he did mention that it's not to be broadcast, I weren't to tell anyone."
He claimed the incident happened at a flat at Grafton Close belonging to John Stingemore.
Mr Stingemore was due to be the co-accused in the trial but died in January.
When asked why he didn't tell anyone sooner the alleged victim replied: "Who would I tell? There was no one else other than the staff. It would only get reported to Stingemore.
"I felt a bit ashamed as well."
Defending the priest Stephen Spence suggested the man was lying.
"You've made this up, haven't you?" He asked.
"No, absolutely not," the man replied.
"None of this happened," Mr Spence added.
Reporters inside court said as the alleged victim walked away from the witness box he made a gun gesture with his hand towards the accused.
Fr McSweeney denies four counts of indecent assault, three of making indecent images of a child and one of taking indecent images of a child.
The trial at Southwark Crown Court continues.