use putty.exe to login to your router.



then on the command line use following commands.



nvram set boot_wait=on

nvram commit

nvram set wait_time="20"

nvram commit



this will give you enough time to upload the image.



open up two terminal windows on your client (this is only windows sample should work with any client you just have to adjust tftp and ping commands)

this assumes that the router has ip address 192.168.1.1



in one window you enter 



ping -t -w 10 192.168.1.1



in the other you prepare your tftp



tftp -i 192.168.1.1 PUT 



go back to your web interface of the router and reboot



as soon as there is no more ping being returned by the router hit enter in the second terminal window and upload the image.