What does it mean if someone tells you not to put your picture up online because others can take your picture and use it to do witchcraft on you?

If someone wants to cast a hex on you, we don't need a photo to do it. As long as there's a clear image in mind and an intent, it can be done. Realistically, though, a witch isn't going to go out of their way to curse a random stranger. That's just asking for trouble.