scrot (screenshot) cron job not executing

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http://askubuntu.com – I'm trying to execute a cron job with scrot (screenshot utility) but it won't work. At first I thought it was a directory problem - I wrote my.png by itself at first, which could have been dropping it anywhere - so I changed the command to use the absolute paths. Since I've defined absolute paths, it seems like it would have to work correctly, but it's not. If I try to execute this as written on the command line, it works. What am I doing wrong? 32 8,14,16 * * 1-5 /usr/bin/scrot /home/johndoe/my.png (note that the times are not the problem, I keep putting it one or two minutes ahead of the c (HowTos)