BBC has changed some of the photos, but the photo in the OP is of Iran sponsored Iraqi militants celebrating as they drive Isis out of Tikrit.
The mask is worn to prevent retribution attacks on family members.
As for who are "good guys," I guess that depends on one's point of view. Isis members are arguably "True Muslims," following the 7th century feudalistic dictates of the Koran...beheadings, stonings and crucifixions included. Also slavery and other barbarisms.
Shi'ite and Sunni have been killing each other for centuries. I'm sure atrocities are being carried out on both sides. I pity all who live in the middle east during this time of conflict.