The images showed Theerawan Charoensuk, 54, holding a hand-sized bowl with a goodwill note apparently signed by former Prime Minister Thaksin Shinawatra, whose powerful political dynasty is locked in a decade-long struggle with the kingdom's military-backed royalist elite.
She faces up to seven years in jail if convicted.
Her arrest comes as the ruling junta, who toppled a government led by Thaksin's sister in 2014, hardens its clampdown on political expression ahead of a referendum on a controversial charter it has penned.
The small bowl is used to pour water during Buddhist ceremonies and was distributed at a temple fair in northern Chiang Mai, the Shinawatra's hometown.
It was painted red -- a colour with strong associations to the family's supporters, who are known as the ‘Red Shirts’.
Local media reported it was signed by Thaksin along with a slogan that read: ‘The situation may be hot, but brothers and sisters may gain coolness from the water inside this bucket’.
Police declined to comment on the details of the bowl, saying only that they believed it was handed out by a ‘certain political party’.
‘She was charged with section 116 -- inciting chaos in the country,’ said Nateephat Akarapongthiti from Chiang Mai's Mae Ping police station.
The junta has outlawed all political activities and protests since the coup, in a vow to bridge the kingdom's vast divides.
But critics say the generals are more focused on rooting out the influence of the Shinawatras, who have dominated electoral politics for the past 10 years and are seen as a threat to the elite's status quo.
Junta chief Prayut Chan-O-Cha told reporters Tuesday he found the charge was ‘suitable’ because ‘it might be a national security threat’.
‘It was [a show of] support for people who have violated laws and run away from criminal charge,’ he said, a reference to Thaksin, who lives in self-imposed exile to avoid a corruption conviction.
Police said Theerawan, a Chiang Mai resident, was released from custody Tuesday and will now face trial in a military court.
‘She said she held up the bowl, but she didn't think that was wrong,’ Nateephat told AFP.
Rights groups say the junta has increasingly leaned on the country's tough sedition law to silence critics.
The charges have been brought against dozens since the 2014 takeover, including peaceful anti-junta activists and pro-democracy students as well as those accused of spreading rumours about alleged military corruption on Facebook.
The military has also overseen a surge in prosecutions for royal defamation, an offence that carries up to 15 years for each charge.