I will post an answer to stop bugging with comments.
Let’s call both parties by their function on this: The NFS server, from now on “the server”; and the image server, from now on “the client”.
When you mount a remote directory, you will see the files on both the server and the client. However that doesn’t mean that the information is stored in both.
Everything uploaded to the mount point on the client is saved on the server (NFS Export).
On the other hand, if you write something on the server (inside the NFS export path of course), you will indeed be able to see it from the client, however it won’t be stored on the client, but on the server.
When you run df -h and it returns root@mainserverIP:/var/www/html/wp-content/uploads 155G 2.8G 153G 2% /var/www/html/wp-content/uploads, you are seeing properties from the server.
If you lost connection to the server, or if you at some point umount the share, you will no longer see that line in df, neither will you have any files or used space in that directory used as mount point.
Hope it helps. Regards.