Initially, you need tftp server (you can refer the link http://www.dslreports.com/faq/9816). Keep in mind that the router and the tftp sever needs to be same subnet and have the same default gateway.
Type below command from ROMMON prompt,
rommon>IP_ADDRESS=10.0.0.1
rommon>IP_SUBNET_MASK=255.255.255.0
rommon>DEFAULT_GATEWAY=10.0.0.1
rommon>TFTP_SERVER=10.0.0.25
rommon>TFTP_FILE=c1700-k9o3sy7-mz.124-5a.bin
You can check that everything is set up correctly by entering set on its own.
Once you are happy then enter tftpdnld at the ROMMON prompt and your IOS image will be installed from your tftp server.
(note: This will erase the flash so any other files stored there will be lost.)
(note: With ROMMON mode case (uppercase or lowercase) is important so make sure you enter command and variables.
References:
http://www.dslreports.com/faq/13824
