There are scripts designed to enhance the Facebook experience, such as:

Always respect the privacy settings of others on Facebook. Trying to access private photos or posts without permission is a breach of privacy and potentially illegal.

Facebook uses server-side permissions. A browser script (Greasemonkey) can only modify what is already sent to your computer. If Facebook's servers haven't authorized your account to see a private photo, that data is never sent to your browser, making it impossible for a script to "unhide" it. Legitimate Alternatives for Viewing Content