erotic photograph; pornographic picture
Refers to sexually explicit or suggestive photographs. エロ is a colloquial abbreviation of エロチック (erotic).
彼のスマホにはエロ写真がたくさん保存されていた。
His smartphone had a lot of erotic photos saved on it.
Compound of エロ (erotic, from English 'erotic') and 写真 (photograph).