To lock your system without pressing alt+ctrl+del
simply go to the desktop
right click and select shortcut
enter the following command
rundll32.exe user32.dll,LockWorkStation
press next and name the shortcut as Lock
press finish and when you click it. it will lock the system.
To change the icon of the shortcut, simply go to the shortcut, right-click it and select properties
Then select the change icon option as shown in the above screenshot
Now type Shell32.dll.
and press enter
You will see the icons as shown in the screenshot below
Now select the icon of your choice and change it