Zainab al-Khawaja, the daughter of prominent rights activist Abdulhadi al-Khawaja, who is serving a life term, was also fined 3,000 dinars (about $8,000/6,450 euros), said the London-based rights watchdog.
Khawaja was convicted of tearing up the photo in a court in October, it said, arguing that this "should not be a criminal offence".
"Amnesty International is calling for this and all of Zainab al-Khawaja's other convictions to be quashed and all outstanding charges to be dropped," said Said Boumedouha, deputy director for the Middle East and North Africa.
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The ruling comes after a court in the Gulf archipelago sentenced her sister Maryam, who lives in exile, to one year in prison for assaulting police officers.
Amnesty said Zainab al-Khawaja was not in court when the verdict was delivered on Thursday as "she is still recovering after giving birth last week".
"If she is imprisoned on the basis of this conviction, Amnesty International will consider her a prisoner of conscience and campaign for the authorities in Bahrain to release her, along with her father Abdulhadi al-Khawaja and all the other prisoners of conscience languishing behind bars," said Boumedouha.
Tiny but strategic Bahrain, home base of the US Navy's Fifth Fleet, remains deeply divided three years after authorities crushed a month-old pro-democracy movement.