is there a possibility to make screenshoots at fixed time intervalls, even if the computer is idle?
There is the "Timed Screenshot Capture" which is every 60 seconds by default. It should still be running even if the computer is "idle" or when no one is using the system. However, if the computer goes into standby or hibernation, then ALL programs will stop running. Although, if you need to for some reason, you can disable the standby on the system by modifying the power options.