It's quite simply to do this without using an external script....
First, keep your cron job....
Second, Create a folder that your cron job will place a single photo/jpg in. This folder must contain only a single photo/jpg.. (Multiple files will give you a random effect, see below).
Third, Open System Preferences -> Desktop & Screen Saver -> Desktop tab
Fourth, either drag or use the + to add the new folder to your list.
Fifth, highlight the folder in the folder list
Sixth, Turn on Change Picture, and choose a time frame.
You could also run a Applescript to reset the picture, but that's the most obvious and simplist way I can see to do that...