You cannot take a picture of a person where the person is the subject of the photo. This is not the case when you take a picture of a statue and there's people in the background, for example.
You can, as long you're not using it for publications. The usage on your own blog or Facebook account is allowed but the line is drawn at billboards, on TV or such (larger) channels. For those you must have an agreement.
This is not true and hasn't been for a while. It used to basically be a grey area but courts have repeatedly stated that just taking the picture was not allowed even if there wasn't ever any intention of publishing it anywhere.
You would have to justify it anyway and personal rights in Germany always trump the rights of others to create art, for example. You'll find many cases that are about surveillance cameras as well and even there's it's often in favor of the person not wanting to be on camera.