You can't!

Facebook won't allow you to hide your profile picture and cover photo (these will be always public, no matter what are you settings). The workaround is to simply change/blur the profile picture, create a fake profile (which isn't allowed) or deactivating your account for some period of time.

On Facebook Help Centre we can read:

Anyone can see your public information, which includes your name, profile picture, cover photo, gender, username, user ID (account number), and networks (learn why).

What information is public?

Information you share that is always public: Some of the information you give us when you fill out your profile is public, such as your age range, language and country. We also use a part of your profile, called your Public Profile, to help connect you with friends and family. Your Public Profile includes your name, gender, username and user ID (account number), profile picture, cover photo and networks. This info is also public. Some of the ways this helps us connect you are:

• Your name, profile picture and cover photo help people recognize you
• Gender helps us describe you (ex: "Add her as a friend")
• Listing your networks (ex: school, workplace) allows others to find you more easily
• Username and user ID (ex: your account number) are in the URL of your profile
• Age range helps provide you with age-appropriate content
• Language and country help us provide appropriate content and experiences

See:

• What is public information?
• How do I control who can see what's on my profile and Timeline?
• (news) Facebook Removing Option To Be Unsearchable By Name, Highlighting Lack Of Universal Privacy Controls

Go to your privacy settings page and change everything to Friends. Each of the sections have areas that need to be changed:

• Default Privacy (within custom, then Friends)
• How you connect (click edit then select Friends for each of the three choices)
• Timeline and Tagging
  • Click Edit then select Friends for each of the three choices with drop downs
  • Turn on "Review posts friends tag you in before they appear on your timeline
  • Turn on "Review tags friends add to your own posts on Facebook"
  • Click "Who sees tag suggestions when photos that look like you are uploaded" and select Friends
• Ads, Apps, and Websites
  • Turn them all to Friends within each of the settings or remove apps that you don't want to use
  • In the "How people bring your info to apps they use" area uncheck everything
• Limit the audience for past posts
  • Click "Limit Old Posts"
  • Set to Friends