Is there any way for uploading to Facebook screenshots that are save on my PS4? I mean, those that I didn´t share at the moment of taking them. If so, how do you do it?
On your home screen, look for the Capture Gallery and launch it. From there, navigate to the screenshot you want to upload; they're categorized by game.
With your screenshot selected (highlit), you can either view it by pressing X, delete or copy it to a USB storage device by pressing OPTION, or upload it by pressing SHARE. The latter is what you want.
All you need to do now, is select where to upload your screenshot. Your options are Facebook, Twitter, or PSN messages.
Image by me. Feel free to use as you see fit.
Right click and select "View Image" to view image in full resolution.