Here is an easy example of how to backup/download an IOS image from a Cisco router.

Need to make backup image? Or need to download image? Try the following:
1) Backup image to TFTP server
Router>enable
Router#copy flash:{your IOS image filename} tftp://{TFTP server IP address}
2) Download image from TFTP server
Router>enable
Router#copy tftp://{TFTP server IP address} flash:
Make sure that you have a working TFTP server before doing the command. If you don't have a TFTP server ready, get a TFTP server software first. Check out this forum FAQ on how to get one for free.