(even though Facebook will just delete this data anyway) You just drag files and drop them onto a script to add that information to them. You can also do this in Microsoft Windows itself... to JPG image files. But these scripts allow you to add data to other kinds of files too such as PNG images, MP4 videos, and PDF documents. Obviously you'll want to edit these scripts in a text editor to use your name instead of mine.
It's easy to view and edit metadata in Windows... when they're JPG pictures. But not with PNG pictures.
But it IS possible to add metadata to PNG pictures using EXIFtool.
I made some easy drag-and-drop scripts that let me add "creator" "copyright" and "comments" to a bunch of pictures at the same time, no matter what format they are. As long as EXIFtool.exe is in the same folder as the scripts.
Many of the pokemon pics were drawn by other people and then colored by me. But they're mostly PNG images. Metadata is a way to sign the work and give credit to everyone who worked on it. It can store things like the artist, copyright, and various comments.