Us the command like this
scp filename loginname@ipaddress:/tmp
eg: scp openwrt-atheros-combined.squashfs.img root@192.168.1.1:/tmp
This will copy the file openwrt-atheros-combined.squashfs.img to the tmp folder in the router
Make sure that you are doing scp from the path where the file is stored or else give the complete path of the input file to be copied.
eg: scp /bin/atheros/openwrt-atheros-combined.squashfs.img root@192.168.1.1:/tmp