don't show contact photos in mail message view
I don't really think the images comes from an LDAP based system.
However, they're likely to come from Contact.app that fetch all users' data saved in it. Mine is full of Facebook people and pictures I see comes from their facebook profile.

Your idea to use the header personnalization might not become handy here because it only allow you to show or hide infomations contained in the mail header. And as I said a little higher, theses images doesn't come from here.

So I don't think there really is a solution to your problem here... Unless you're ready to refuse Contact.app from fetching infos on G+, Fb, LDAP etc.
Or you could use another mail application