Q: Disk Space

There is no secure empty trash in El Capitan.

Note that if you deleted items from iPhoto you have to empty iPhoto's own Trash and if from Photos you have to remove recently deleted items.

You can use an app like the following to better determine what is using storage so you can delete it

http://grandperspectiv.sourceforge.net/

https://www.omnigroup.com/more

Also, if there is a lot of "other" see:

How do I get rid of "other" storage

Note that some links may not work because of a temporary problem with this forum

Did you empty the separate Trash in the iPhoto app if yo used iPhoto?

Did you remove recently deleted items from the Photos app is yo used that app?

Those are separate from the mac's general Trash

For other:

https://support.apple.com/en-us/HT202867

What is "Other" and What Can I Do About It?

If these images were being used in iPhoto and imported into Photos, then they would have been "hard-linked" - meaning that there was only one copy of a file being linked from both the iPhoto an Photos libraries. This would allow for, saying, duplicating an 800GB library without using more space; it would also explain why deleting one reference would not recover any space, since the files would still be referenced once.

Did yo see the two links in my second replay about "other"?

There will always be some other.

How much "other" do you have?