On the one hand, I can understand keeping pictures of you off of the Internet. Once it's on the net, it's there forever. It will never be completely deleted.
On the other hand, my sister was this sort of a prima-dona growing up as a teen. We would have to sneak pictures in. She would throw fits. And some of the only pictures we have of her as a teen were snuck in and look awful. If she would have just grown up and let people take pictures and smiled nicely then we would have had some good pictures. See, I agree with your right to privacy, but I also believe your family has a right to have pictures of you. You're changing every day, every year, and just because you don't like pictures doesn't give you the right to rob people of those memories and the pictures to help. Also, there are times when emailing pictures or using a photo-site to send to family can really be helpful.
My sister has gotten over it. She looks at the pictures now and realizes she didn't look nearly as bad as she thought she did, and that we wanted to remember how she looked because we loved her. (The only exception was when she shaved her head, we took those pictures to laugh later. But if you're going to let your 14 year old friend shave 'steps' in your hair, then that IS cause to laugh...)
Now, how do you get them to understand? I would say that first they would have to feel that you're not going to take away the pictures from them. And then just keep repeating yourself. Say that if they want to send it to grandma, use email. That you have 'an online stalker' that you really don't want them to have pictures of you. And then keep on it. I don't like anyone but ME to put pictures of my kids or myself on Facebook, and my family knows why (I had identities stolen when I got pregnant with my first, some stupid teen who wanted attention.) If I know that all pictures originate with me, then if I see a picture of my kids somewhere then I can get the webmaster to delete it as 'owner' of that picture. Not everyone agrees with me, but most are respectful at this point, but it took a couple of years and there's still one who ignores me.