In Facebook, does sharing pictures of great personalities with page watermarks on them create a legal problem?

You said you share “copyleft picture quotes” but then you apply your page watermark. It sounds like you could be heading for trouble.

A “copyleft” is intended to be the opposite of a “copyright.” Instead of all rights to the image being reserved, on the contrary, all rights are freely given to redistribute the image, so long as any derivative works are also freely given.

So why a copyleft instead of just releasing something to the public domain?

In the public domain, anyone can similarly use the image. But public domain works can be re-packaged into proprietary works, which can then be subject to copyright.

Copyleft works are specifically prohibited from being re-packaged into any derivative work for which a copyright can be applied. Copyleft says that anyone who redistributes the image, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.

If the original image is subject to “copyleft,” you are free to redistribute it, re-package it, and even modify it, so long as the same “copyleft” applies to the image.

In other words, you cannot watermark the image unless it is a “copyleft” watermark. You cannot restrict redistribution of a copyleft image by placing your own copyright on it. This violates the letter and the spirit of a copyleft.