I have discovered my photos that are saved on Google Photos will copy and paste straight into the forum, easy peasey.
Google Photos will definitely not work!
May be for you, because the picture is in the cache of your browser but not for the others.
External linking was possible and allowed in
Google Picasa (which is frozen now, so no new updates). With the change to
Google Photos is was explicitly stated, that external linking of the pictures is not allowed and/or possible (help pages). The links are changed often.
You can only generate links for sharing, but the users have to visit the Google pages to see the photo, so this is definitely not a option here in the forum.
Suggest to follow the recommendation of Gary.
Cheers,
Frank