Problem with Crons

[Bellardia]

I made a script that uses image id's stored in MySQL to create custom images, save them to a file, and then put the file url back into the table.

The problem comes from the cron job. I've set it to run every 10minutes, which it seems to do just fine, however when it runs on its own the result is an empty file consisting of just the background layer. The thing I don't understand is that if I go to vbulletin options > scheduled tasks, and 'Run Now' the images are created perfectly.

Anyone know a possible cause for this problem?

Thanks a lot!