In case anyone is interested, here is a legal article about jurors accessing social media during trials. In my opinion, the overarching principle is that jurors shouldn't read, say or do anything that could affect their ability to bring an impartial mind to assess the evidence before the court. If there is any risk of those two things (impartiality and evidence before the court) then the offending juror should be discharged. The Judges directions would be relevant. If he told them she's should not access the Internet during the trial, then any internet access at all would be viewed with suspicion. If, however, his directions were to avoid discussing the case, and the juror has just posted a photo irrelevant to the case, on Social media, then there is a chance that the juror hasn't breached the two tenets. This would involve very significant questioning of the juror about what else they said, saw, did etc.
If a juror just went online to do some banking, then a judge could probably accept an assurance that nothing was seen or done to taint the juror. But given the nature of social media today, i think a judge would be sceptical about a juror claiming they saw nothing about the trial while on Instagram.