No (free)tags in Tiki 25, files gallery ?
Ha ha...
I see Luci posted a workaround;
It is currently (21.x) only possible to tag files using the Freetag module which has to be assigned to a module zone.
Still unsure how simple the could be, but yeah look like a missing feature...
One can imagine we could use trackers to refer each picture as a trackeritem... but it will make things much more complex (from a photographer point of view).