To add a chrooted FTP user outside of Plesk properly, you need to:
Create the user with the home directory as the root of what they can access
Give the user a password
Make their primary group psacln
Add them to the psaserv group as well
# useradd username -d /var/www/html/website/slideshow/
# passwd username
Enter new password for username
# usermod -g psacln [...]

As you may have found out, a web server that houses a bunch of web sites can cause some issues with file permissions when it comes to allowing PHP to write files that a web site needs, whether it is a data file, a document or an image. This is usually not an issue [...]