Your logic here is flawed. I will ignore the people that are just security minded, as that was already addressed.
People with a criminal background and/or people doing bad things have no reason to hide their martial arts training. If anything, they may want their picture taken so that they can brag about it. The people who may want to have their picture taken is the people being abused, or past victims. They likely won't want their former/current attacker/abuser to know where they are, and a picture automatically tells them that. It also alerts people that they're training, which may piss off the person abusing them (as he/she wouldn't want their 'prey' to be able to fight back), and prevent them from leaving safely. It also can cause issues if someone has a stalker they're concerned about, which again would be a reason for them to train to begin with.
On a less extreme, but personal/anecdotal level, I worked with substance abusers who were on probation or parole. Some undoubtedly blame me for them ended in prison, as I informed their parole/probation officers of their drug use, attendance, and crimes I caught them committing (all of which they agreed I would tell their PO on sign up). As a result, I kept my social media barebones, and gave little information about myself/my whereabouts online. I would not have joined a school if I knew they were posting pictures of me training, and I could not opt out. Now, having left that field, and especially having left that state, I don't care.
So by taking requiring consent for pictures upon signing up, you are not keeping the bad people out, but their victims/potential victims.