You have (best USB flash drive or disk) to create the Fritz box the required folder in the storage medium itself, because the software of Kamerea this is not apparently able.
Here I describe the procedure for the Foscam firmware version 1.4.1.9 with the application number 2.14.1.10.
For this you have there only a folder with the camera ID edit.
These can be found under [Config> Status> Device Info].
This folder, in turn, must be created a folder 'snap'.
Then put in the configuration of the camera the FTP path on the Fritz box with the required path to the folder that contains the device ID.
According to his access code to Fritzbox.
Then it works with actually saving the alarm images in the Fritz box.