Haseeb Hamayoon, 28, sent the image to friends and a co-accused branding the officer "kaffir" - non-believer - London's Woolwich Crown Court was told.
Prosecutors say Mr Hamayoon, Yousaf Syed, 20, and Nadir Syed, 22, plotted an attack using a Rambo-style knife.
The cousins deny all charges.
The image, allegedly taken on 19 September 2014, shows the back of the officer and was apparently taken by someone in a car.
Mr Hamayoon wrote: "Look at this kaffir. Hounslow parking lot."
Nadir Syed wrote back: "How can you pray in such a place?"
Mr Hamayoon replied: "You're right I'm walking out. This kaffir cop yeh.  Looking inside every car who's in there. I gave him a dirty look."
Mr Syed replied: "He's looking for u mate. Mi5 sent him to keep eye on y."
The messages were exchanged on encrypted messaging service Telegram.
Prosecutor Max Hill QC told the court: "You'll see on Mr Hamayoon's mobile phone two photos taken that show a police officer or PCSO walking away from the car and around a car park.
"The photos are taken from the driver's seat of the car. Those images are created on the telephone. It's quite clear what Mr Hamayoon was writing messages about."
Two days later, IS militants issued a "fatwa" calling on followers to attack Westerners. It implored them to: "Rig the roads with explosives for them. Attack their bases. Raid their homes. Cut off their heads."
Prosecutors claim the trio plotted to carry out a terror attack in the UK.
Mr Hamayoon and Nadir Syed, from Hounslow, west London and Yousaf Syed, from High Wycombe, Buckinghamshire, filmed themselves stamping on a Remembrance Day poppy and were obsessed with the killing of Drummer Lee Rigby, it is claimed.
The trial continues.

The incident happened in December when the 18-year-old man suffered what was described at the time as a broken jaw.
Police want to trace a woman and a man.
The woman is described as being in her late teens to early 20s, with long brown hair. The man is thought to be in his early 20s.
The woman was wearing a beige-coloured jacket and beige knee-high boots.
The man was wearing a dark blue puffy jacket, blue jeans and grey and black trainers.
PC Graham Crawford said: "This is a serious incident resulting in a male sustaining a severe facial injury.
"Violent behaviour will not be tolerated and I would urge anyone who recognises the male and female pictured, or has any knowledge of this incident to come forward and contact the police."

Eight-year-old Garfield, who makes the trip to Sainsbury's in Ely in Cambridgeshire on a daily basis, has his own Facebook page created by fans.
Owner David Willers posted a message asking shoppers to lay off the treats as a vet found Garfield, who weighs 6.6kg (one stone), was overweight.
"Everyone loves him," Mr Willers said.
"The other day when it was cold and he stayed indoors, a member of Sainsbury's staff messaged me to ask where he was, which was very sweet.
"He brings more customers into the shop - people tell me they go and shop there just to see him."
Garfield's Facebook page was set up last summer by a couple who had spotted him in the shop, and is now run by Mr Willers.
"We worry about him crossing the road, but we don't really get any negative comments about his habits," he said.
"We live about a four-minute walk away from Sainsbury's but Garfield knows all the shortcuts so can get there quicker than that."
A supporter of so-called Islamic State photographed a police officer in a mosque car park before plotting to carry out a knife attack around Remembrance Sunday 2014, a court heard.
[NEXT_CONCEPT]
Police investigating an "unprovoked serious assault" on a teenager outside Stonehaven railway station have released CCTV images of a woman and man they want to trace.
[NEXT_CONCEPT]
The owner of a cat which visits a supermarket every day, has had to ask people to stop feeding his pet because he was putting on weight.