Boot System Commands

Router(config)#boot system flash image-name	Loads IOS with image-name
Router(config)#boot system tftp image-name 172.16.10.3	Loads IOS with image-name from a TFTP server
Router(config)#boot system rom	Loads IOS from ROM
Router(config)#exit
Router#copy run start	Saves running-configuration to NVRAM. Router will execute commands in order they were entered on next reload

If you enter boot system flash first, that is the first place the router will go to look for the IOS. If you want to go to a TFTP server first, make sure that the boot system tftp command is the first one you enter.