I managed to solve the issue, here is my documentation.
Perhaps usefuls as a base for another FAQ.
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How to organize the piwigo data directories to reside on a nas.


*ip_addresses and user--accounts have been changed in the doc below.*

My setup is like this:
My nas is a Synology d214+ located on ip-address 10.31.1.60
On that nas i have defined a guest user with password guest

On that nas i have created the following shared folders:
piwigo
piwigo_upload
piwigo_data
The guest user i have given read/write access to all 3 folders

My webserver is a Linux Clearos 7.10 system.
On that system, i have created the following directories:
/mnt/piwigo
/mnt/piwigo_upload
/mnt/piwigo_data
Initialy i have given these directories 0770 permissions

Next I had to adjust the file /etc/fstab on my webserver
I added the following lines there:
//10.31.1.60/piwigo     /mnt/piwigo  cifs auto,rw,username=guest,password=guest 0 0
//10.31.1.60/piwigo_upload     /mnt/piwigo_upload  cifs auto,rw,username=guest,password=guest 0 0
//10.31.1.60/piwigo_data     /mnt/piwigo_data  cifs auto,rw,username=guest,password=guest 0 0

After saving the file, i ran the following command:
mount -a
Now the folders on the nas are available via the webserver.

Next step was to rename the original piwigo folders
I had piwigo installed in the following directory:
/var/www/html/piwigo

the following directories were renamed:
upload ==> upload.old
galleries ==> galleries.old
_data ==> _data.old

Final part is to put the sym links in place.
In the terminal window you need to give the following commands:
cd /var/www/html/piwigo
ln -s /mnt/piwigo ./galleries
ln -s /mnt/piwigo_upload ./upload
ln -s /mnt/piwigo_data ./_data

That should do the trick.
If all is working fine, you can delete the folders you just renamed.