Among all the other secrets Facebook have easy tricky secret inside that you can make your timeline profile picture un-clickable for everyone who is viewing your profile. So, when someone click on the thumbnail of your timeline profile picture nothing will appear so your full sized profile picture will be unreachable.
How to Make Profile Picture Unclickable for friends and public
Step 1: Go to your Timeline Profile and click on Photos.
Step 2: Now from photos select albums and Profile Picture.
Step 3: On the page select your current profile picture and set its privacy to "Only Me" from default "public".
Now your profile picture will be unclickable for everyone even your friends.