Disk space not free up after photos and videos are deleted from Photos app
Your pictures didn't get deleted because they still exist in iPhoto or Aperture, whichever one you use.  As Apple says in this support article, Photos saves disk space by creating links to the already-existing files in iPhoto/Aperture, rather than duplicating them. That means that if you want to delete a file, you need to make sure it's deleted everywhere.

If you can live without ever using iPhoto/Aperture again, go to your Pictures folder and delete iPhoto Library and Aperture Library. That way, the only copies of your pictures will be the ones in Photos, and deleting them will free up space on your computer.