If you can prevent either, you should prevent both.
Your reply implies that the child has been taught why it's just as bad to send a picture of their feet as of their hair as of their ankle as of their entire body as of anything. Or that they have parents who wouln't agree to them doing things that seem totally reasonable to them, and just freak out without any explanation.
Children are taught what nudity is very early. They aren't taught people's weird kinks. The difference between the two cases I mentioned is much greater than that between fentanyl and heroin. Both are not ok, but they are not even close in terms of possible adverse effects. You can't judge everything assuming the child has knowledge of the intentions of the abuser online. If your child sends the pictures because 'it's just feet, it can't hurt me' that's a much better reason, even though the child hasn't finished developing it's brain than 'my dad doesn't want me to send any pictures'.