Can I be fired from my job for sending a naked picture to an employee on my off time?

This is sexual harassment, and you can certainly be fired for it. I'm sorry this happened.

When one employee makes an unwanted sexual overture to another employee, that is sexual harassment. So, to answer your question, yes, sending a naked picture of yourself to another employee is a sexual harassment issue, and an employer may, under general principles of law, fire its employee for doing so.
Not legal advice, just general information. I'm not licensed in Texas and don't practice law there. If you need legal advice, please consult a lawyer who holds Texas licensure.

Yes, you can be fired for sending naked pictures on your off time. In most states, absent discrimination, you can be fired for any reason. It would be sexual harassment, however, your employer resolved the problem by terminating you. I'm sorry this happened to you, but employers are required by law to deal with sexual harassment.