Juan Maria Ojeda, an official in Borja town, said 80-year-old Celia Gimenez decided to touch up the fresco of Christ wearing a crown of thorns in the Misericordia church because she thought it needed restoration.
He said no one realized how badly disfigured the painting was until she rang town hall to say what she had done.
The fate of the painting has made national news in Spain.
The fresco is of the genre known as "Ecce Homo" style ("Behold the Man"). But on Thursday some Twitter users were dubbing it "Ecce Mono" ("Behold the Monkey").