Well, to start with you should test it by typing in your browser the full url:http://www
(by default this would be: http://www
but in your case you have used the folder name: img )
and it should run fine (with no output) and email the backup to you, so long as you have configured it corrrectly, if not this should show you the error.
If that runs OK, stop wasting your time with cron jobs and use phpJobScheduler to run the backup:http://www.dwalker.co.uk/phpjobscheduler/