This picture was seen by many around the world. Apparently, it shows Basij members bringing a truck full of bricks and stones to help them attack the embassy more efficiently. Rajanews, a website close to the attackers, gives a “convincing” story about the truck,

While the students were attacking the embassy, a truck full of bricks, stones, and soil passed by. The students used the load of the truck to throw at the embassy.

While it is obvious that with the Police presence around the embassy (see) it is not imaginable how that truck might have reached there, at the exact time it had been needed, Azadeh and I are almost sure that, according to the law, no truck is allowed to travel inside Tehran in daylight. The conclusion is, guys brought their weapons themselves.