In order to back up the IOS you need a TFTP server, SolarWinds offers a Free TFTP Server. If you have no idea what version of the IOS you are using use the show flash command.
Router#show flash
System flash directory:
File Length Name/status
1 3289170 d1206.bin
[3289236 bytes used, 905068 available, 4194304 total]
4096K bytes of processor board System flash (Read/Write)
From the console in privileged EXEC mode enter the copy flash tftp command. When requested enter the IP address of the TFTP server.
Router#copy flash tftp
Source filename []? d1205.bin
Address or name of remote host []? 192.168.1.21
Destination filename [d1205.bin]? y
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
3289170 bytes copied in 47.668 secs (69982 bytes/sec)
To copy the IOS from a TFTP server use the copy tftp command. When requested enter the IP address of the TFTP server containing the IOS.
Router#copy tftp flash
Address or name of remote host []? 192.168.1.21
Source filename []? d1206.bin
Destination filename [d1206.bin]?
Accessing tftp://192.168.1.21/d1206.bin…
Erase flash: before copying? [confirm]
Erasing the flash filesystem will remove all files! Continue? [confirm]
Erasing device… eeeeeeeeeeeeeeee …erased
Erase of flash: complete
Loading d1206.bin from 192.168.1.21 (via Ethernet0): !!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
[OK - 3289170/6578176 bytes]
Verifying checksum… OK (0xB6BD)
3289170 bytes copied in 89.272 secs (36956 bytes/sec)