Sorry for this stupid question.
With the admin account, few categories have been created like 'Travel'.
How could a registered user can put a photo to the 'Travel' catgeory after upload: using key words ?
Many thanks,
You have to create an album inside that (or instead of that) category first.
Set the albums permissions to "upload" (Album settings)
and give your registered users the right to upload (Group settings).
Thanks, It's working now ;)!