If I set a photo to private can only friends see?

yes

No one can see them unless you send them the link
for the 'Private' images as follows:

How do I make Photos 'Private?'
When you place a picture in the 'Main Pic' slot
it will allow you to designate it as "Public"
or "Private."

You do this by left-clicking on the photo.

To give someone access to the 'Private' images,
you merely right-click on the 'Private' photo
which gives you a drop-down menu, left-click on
'Properties' and copy the URL (link) of that
photo and send it via email to anyone you wish.

It stays 'Private.'