Can a former police officer take pictures of you?

Former police officers don't have any special rights in this regard, but that still means the answer is "yes," since basically anyone can take pictures of you.

There are some limitations on how the pictures can be used ... people are generally considered to own the rights to their own likeness, so you usually can't use a photograph of someone for commercial purposes without their consent (with some exceptions if the subject is "newsworthy"). But the act of taking a picture is not illegal, nor can you sue over it.