To answer the general question, two ways to do this, put it in a while/sleep loop or use a crontab
1) while/sleep
#!/bin/bash
while true; do
cp -f /customTemplates/login.tpl /www/img/templates/adm/login.tpl
sleep 60
done
2) crontab (preferred)
Run crontab -e
and put the following line there
* * * * * cp -f /customTemplates/login.tpl /www/img/templates/adm/login.tpl
This will run the command every minute of every hour of every day of every month of every day of the week. (ergo every 60 seconds)
But, as Aaron Digulla said it would be better to get to where it's pulling the config from and edit it there, rather then overwriting it every 60 seconds.