Since long time now I do not lock anymore changes when doing the backup
I was doing that so all pics are exactly corresponding to the database backup. Now at each backup I do manually I take the nightly backup and copy the pics in the day, so in case I need to recover from it there may be a few pics too many or replacements missing (those made in the day of the backup), but not a big deal, as it is a "worse case situation" anyway.