The trial in Frankfurt is the first in Germany for war crimes committed over the course of Syria's five-year war.
The photographs in which Aria Ladjedvardi appears were taken in the spring of 2014 and posted on Facebook.
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Federal prosecutors believe he and two fellow fighters took the pictures to belittle their victims, whom they considered infidels.
Ladjedvardi, a German of Iranian origin, was arrested in October 2015 in the Frankfurt region, after police raided his apartment. He has since been in custody ever since.
He travelled to Syria in early 2014 to join one of the militant groups engaged in the fight against President Bashar al-Assad's regime.
In Syria, he received weapons training, according to prosecutors.
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German federal prosecutors are currently investigating 10 cases of alleged atrocities in Syria or Iraq, along with more than 30 cases of suspected membership of a terrorist group involving militants returning from the Middle East.
The investigations have gained momentum with the arrival last year of over a million asylum seekers, about 40 percent of whom fled the wars in Syria and Iraq.
Authorities dealing with their asylum requests have picked up and forwarded on average of 25 to 30 tip-offs a day to prosecutors, in line with a German requirement that asylum applicants provide information on any war crimes they may have witnessed.
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