There is a plugin on Piwigo called "Add Index" that will help some, after you install it go tools/maintenance and pick Add and overwrite "index" files for all local sites

also if you have access to some cpanel /server options you can try setting uploads directory to "no-index"

as well in your robots.txt you can place the lines below for google and other good bots to follow.

# robots.txt file Please Obey
User-agent: *
Disallow:
Crawl-delay: 5
Disallow: /upload/

Last you can setup a php trap file for bad bots which makes it so when they scan the upload folder
they read a php file that adds their ip to the .htaccess file and prevents them from accessing the site again. That would be for the bad bots.

Hope this helps