If you wish to change padlock image on Joomla Administrator Page, you can change this with some steps :
1. Open Administrator directory
Joomla 1.5 :
/administrator/templates/(name of template being used)/images/j_login_lock.jpg
Joomla 1.7-2.5
/administrator/templates/(name of template being used)/images /j_login_lock.png
2. Open and edit the image with exact size and save on its place
