Generally we advise not opening and messing around in the Photos Library-- it can irreversibly corrupt the Library.
It's not clear what users you are talking about, or what you're exporting from or to. iCloud? New user? Old user? Are you using "Optimize Storage?"
Photos only shows pictures that are in its local Library--pictures at iCloud must transfer to the local library to be used by Photos. If you see pictures in Photos, you are looking at images that are in a Library on the computer. But there may be more than one Library.
A Library in the Pictures folder owned by User#1 is not available to User#2, and vice versa. If you try to look, you get this:
The only way you can even tell if there is a Library in another user's folders is to sign in as that user.
You can see all the Libraries that you do have access to by opening the Library Chooser. Close Photos and Option-click on the Photos icon to get this:
This will show the path to each Library, and it identifies the System Library. If the Library you're using isn't the one you want, you can change it here.
If you can see pictures at iCloud that you don't see in your local Library, then that Library must not be connected to iCloud. If you want the pictures at iCloud.com to be in this Library, then you can check Photos' Settings>General to see if this Library is the System Library-- only the System Library can connect to iCloud:
"Use as Photos System LIbrary" is grayed out here, because mine already is.
Then you can check Settings>iCloud to make sure this System Library is connected otherwise iCloud. Once connected, pictures in your local Library will be copied to iCloud.com, and those at iCloud will be copied locally.
Let us know how this is going, and if it's making sense…
