Under any circumstances, DO NOT delete working IOS image off the router flash memory unless the image has been backed up to server (TFTP or FTP server) or you have working IOS image stored in the server. Later description will show you how to back up the image
* There is a chance that the router loses power or loses connection during the IOS image transfer (copy from or to server) which may cause the router to have no working IOS image to boot up from. When such incident happens, you will be glad that you first back up the IOS image to server before deleting working IOS image off the router flash memory.

Quick Guide

Need to make backup image? Or need to download image? Try the following

Backup image to TFTP server
Router>enable
Router#copy flash:{your IOS image filename} tftp://{TFTP server IP address}

Download image from TFTP server
Router>enable
Router#copy tftp://{TFTP server IP address} flash:
For those routers or switches that have USB port, you can use USB Flash drive to backup or to download IOS images as follows
 

Backup image to USB Flash drive
Router>enable
Router#copy flash:{your IOS image filename} usbflash0: