is it cheating?

Sexting or sending nude photos is a violation of the emotional contract you have with your partner. Flirting is different than teasing, which some people don't understand. When you send a nude photo of yourself, you want the receiver to admire you, or desire you sexually. The only person you should need wanting you in such a way is your partner. So yes, it is considered cheating. If you have the urge to cheat, perhaps you aren't ready to be in a relationship. If you want to send/receive nude pictures or, have sexual contact with other people than your partner, then end the relationship and do as you will. No need to break a heart.