Piwigo/language/en_UK/help/help_virtual_links.html

<p>Photos are displayed in the album corresponding to their file
directory.</p>

<p>You can also display a picture in several albums, without duplicating
the file on your web server. Associate the photo to any album from the
photo editing screen (that any administrator can reach from the photo page
on the gallery side).</p>

<p>Virtual albums are based on this concept: they have no corresponding
directory. Create virtual albums
from <span class="pwgScreen">Administration &raquo; Albums &raquo;
Manage</span>.</p>