I've been experiencing the same issue.  Generating an image sometimes takes a long time (for me I blame it on the NAS CPU and lack of ImageMagick).  Normally I see it a few hours or a day later.

To debug your issue turn on logging:
1. read this article http://piwigo.org/doc/doku.php?id=user_ … onf_locale (it updates ./local/config/config.inc.php)
2. add this line to the file $conf['enable_i_log'] = true;
3. save and close
4. kick-off a process that will now generate log data

The log file is located at   ./_data/tmp/i.log

After you have logging turn on go back to the same page where the image is missing.  You should then be able to see something written to the log file (./_data/tmp/i.log) showing that the image is being generated.

If nothing appears in the log then try to use the Batch Image generator to force the process. 

If still not getting anything in the log or on the page then the issue is deeper and will require debugging of your system.

P.S.  Also check your cache folder to see if an image was created './_data/i/<site_name>/<album_name>/'

Last edited by ct27 (2016-01-06 18:00:29)