The i3-wm does not come with a keybinding to lock the screen or a preconfigured auto lockscreen. This gist describes how to setup both using i3lock and xautolock. i3lock is a minimalistic lockscreen and xautolock monitors mouse and keyboard activities to automatically lock the screen after a certain time of beiing inactive.
First get the tools if neccessary.
E.g. sudo apt install i3lock xautolock
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To setup the keybinding Ctrl+Alt+l (last one is a lowercase L) to lock the screen append the following lines to the i3 configuration file located at ~/.config/i3/config
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# keybinding to lock screen
bindsym Control+Mod1+l exec "i3lock -c 000000"
-c 000000
makes the screen turn black instead of the default white after the screen is locked.
Then add a line to the same config file to run xautolock at startup.
# auto lock the screen
exec "xautolock -detectsleep -time 3 -locker \"i3lock -c 000000\""
-detectsleep
locks the screen properly when the computer goes to sleep, -time 3
sets the time after which to lock to 3 minutes and -locker "..."
sets up i3lock as the command to lock.
Finally reload the config file via $mod+Shift+r
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i3lock does not have an image scaling feature to my knowledge hence we need to convert the image to monitor resolution (1920x1080 in this case) and use that new image with i3lock. I assume you've already found the solution. If not, you an use this method.