Starting with 14.1RC1, the Image lightbox was added as part of XWiki, as disabled by default. It was done like this initially since it was new (to have it tested first) and also because it could have interfered with features that display images for other purposes. But this was solved with [XWIKI-19436] Add the possibility to disable the lightbox feature for specific content - XWiki.org JIRA and some other problems were fixed as well, making the lightbox more stable.
Considering this, I propose to make it now enabled by default, since it’s a nice feature and it would be easier for users to discover it.
It can also be easily disabled from the Administration section