From the article the OP linked:
WCCO spoke with law professor Rachel Moran from the University of St. Thomas about what this photo could mean for the case.
“Did the juror speak the truth? Or alternatively, did the juror say something untrue during questioning?” Moran said. “But the other thing to keep in mind is did the lawyers do their job in investigating the juror?” Moran said.
Before being selected, Mitchell filled out this questionnaire. On it, he said he never attended protests over police brutality in Minnesota or beyond. He also answered a question on Black Lives Matter, saying, “Black lives just want to be treated as equals and not killed or treated in an aggressive manner simply because they are Black.”
“If he had been asked about it and he tried to hide it, that could be an issue,” Moran said. “But at this point, I don’t see anything, any evidence that he tried to hide it.”
The defense does now have the right to ask Judge Peter Cahill to go back and question Mitchell, then Cahill can decide if the verdict will stand.
“I think it’s really important for the viewers at home to know it’s really hard to overturn a conviction, and courts are especially reluctant to interfere with the jury deliberation process,” Moran said.