Hi,
I understand that you still wants to track your 500 employees even if they turn-off their data plan.
Actually there is a way, especially if they are always on places that they frequently go to.
What will happen here is that during the day, the coordinates of the GPS will be stored in the mobile device, and when they get online at the end of the day, those data can be sinked to, let say a remote server where you can have control of each one of them.
You're approach is okay, but knowing the purpose of downloading those images will prompt them and they will gather all excuses not being able to download them in time... one reason is that they need to turn on the data plan to be able to download it, second one might be they are busy that time that they can't download it and unfortunately forgot about it afterwards.
I can give you alternative solutions to your problem.
Let's talk.