How do I share a picture from Photos to Facebook?

To upload Photos you do not go to the Pictures Folder.

When the file chooser dialog appears, when you are trying to upload photos, you scroll down to the Media section in the sidebar:

Click "Photos" below "Media", then select the Photos Library, when it appears. To the left of "Photos" will be a disclosure triangle. Click this to reveal the albums inside. This should work, when you try to upload to Facebook using a web browser.

It is not possible directly from the Photos.app on Catalina. Apple has summarily removed the built-in sharing options for social media sites ( Facebook, Twitter, LinkedIn, Flickr, and Vimeo) from MacOS X for security reasons when macOS 10.14 Mojave came out (https://www.macrumors.com/2018/06/05/macos-mojave-security-privacy/).

The new method is safer. Instead of the old method, where Apple managed the social media accounts in the System Preferences > Accounts on our computer, we now have to install a sharing extension provided by the social media site. You can still share photos directly from Photos, if the site is offering a sharing extension (we are still waiting for an extension provided by Facebook).

See: Share photos using other apps and websites in Photos on Mac - Apple Support

Do it other way round. Open your facebook page in a web browser, then open the photo browser to upload some photos to a Facebook album. In the File chooser dialog scroll down until you are seeing Media section in the sidebar. Click "Photos" in "Media", then select the photos library.

if you are using the Media Browser to access your Photos Library from the photo uploader on a web site, view the photos enlarged in Photos, before you open the web page. By viewing the photos enlarged, you can force the download from iCloud and make the photos locally available for sharing.

Yes, you cannot access Photos Library from the Finder. The Photos Library is a package - a complicated structure of connected databases and media files, and you need a program to find the photos there. The Media Browser (in any file > open dialog) can show you the photos there, and also the Photos.app.

You will see, why the Finder cannot show you the photos inside the library, if you look at the package - but be very careful and do not modify anything:

• Open your Pictures folder and select the Photos Library.
• Ctrl-click or right click the library.
• You will see a contextual menu. Click "Show Package Contents".
• The Finder window, that will open will show you the files and folders in the package. Your original image files are in the folder "Originals", renamed and hard to identify, and stored in a random fashion.

It is simply not readable for humans. Close the window without changing anything.

You are welcome, simeon 😊