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.
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.