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You can never use tunneled mode for the OS image management.
What is Tunneled mode? Tunnel mode is simply utilizing the SSH/Telnet session to collect or deploy data to the device. Or more specifically we issue commands to display data on the SSH/Telnet terminal screen and then the application "scrapes" the data from the screen. Example for the snapshot running the application performs the "show running config" command. The device then displays the running config to the terminal "screen" and TSNA then collects the information.
Since you cannot actually use the SSH/Telnet session to transfer a binary file like an OS image file tunneled mode cannot be used for these actions.
Please let me know if there is any additional questions or confusion on this that I can clear up.