The pictures published by Mr Heydarian were taken at the LidoMa Festival of Iran’s Lur people
The poisonings had roiled the country and sparked new protests last week.
There have been more than 3,100 reported cases of poisoning involving more than 100 schools, according to Iranian media.
In recent weeks, numerous Iranian journalists have been detained as part of a wave of arrests.
22-year-old Mahsa Amini was arrested and allegedly tortured to death by Iran’s morality police for “unsuitable attire”.
Talks on reviving the 2015 nuclear deal between Iran and the six other parties to the agreement have been deadlocked since March.
Foreign Minister Hossein Amir-Abdollahian says Iran will reopen its embassy in Riyadh.
Hassan Sayad Khodai was gunned down in broad daylight in front of his home in central Tehran on Sunday.
The new sanctions came amidst international efforts to reach an agreement with Iran to revive the JCPOA.
Negotiations on reviving the 2015 Iran nuclear deal will resume on Monday in Vienna amid few signs of progress and a tense atmosphere.
