The images show four men dressed in black kneeling in an empty street with knife-wielding militants standing behind them, after which they are pictured being beheaded.
Text accompanying the photographs said the beheadings were carried out in Salaheddin province, where Iraqi troops backed by militia are fighting to retake the provincial capital Tikrit from IS.
The exact location where the photos were taken was unclear, and their authenticity could not be independently confirmed.
IS spearheaded a sweeping offensive that overran large areas north and west of Baghdad last year.