With lxsession git master (2016-11-27), notice the following:
1) Open lxsession-default-apps, and Image viewer will probably be disabled.
2) Choose an application and exit. Restart the session.
3) Start lxsession-default-apps again. Image viewer is again disabled.
The cause is simple: there is an inconsistency between the use of "image_display" in lxsession-db and lxsession-default-apps, and "images_display" (in the plural) in desktop.conf, lxsession proper, and lxsession-default. Even though I think the singular makes more sense in context, given that the latter is public, should the former be changed to comply?
Thank you very mcuh for reporting this. It makes sense.
Fix pushed into GIT, it should appear in next release.