1:37 PM PT -- The secret service just confirmed TMZ's story that the guest in question did not breach security by having his pic taken in the Mar-a-Lago study. The Secret Service just said all guests had been fully screened and could freely enter that area.

Donald Trump's Secret Service did not screw up Saturday, despite the claim of a Mar-a-Lago guest.

Joseph Young posted a pic on IG mugging in front of a Trump portrait in the study. Young, a guest at Mar-a-Lago, captioned the photo, "Snuck by secret service to catch this selfie. They might have told us not to go in there."

A Secret Service source tells us ... guests and members were indeed allowed in the study where the pic was snapped. Trump has been at Mar-a-Lago all weekend.

Another guest, who says his name is Frank Vigilante, posted a pic of him and his mom standing in front of the Trump portrait with the caption, "In Trump's study with Mom."

There have been multiple incidents at the White House over the last week with would-be intruders ... the most troubling being a man who jumped 2 fences and went undetected for nearly 20 minutes -- getting right up to the building and jiggling a doorknob.