This issue may be caused by a problem with the TFTP server program or the image may actually be wrong for the router.
If this error message is displayed while attempting a Cisco IOS® Software upgrade through TFTP, verify that the image being copied is correct for the hardware.
Perform these steps:
%Warning: File not a valid executable for this system
Abort Copy? [confirm]n
Note: This should cause no problem as long as the image is actually the correct image for the hardware.