Resize image file on Ubuntu/Mint in console:
sudo apt-get install imagemagick
convert <input-file> -resize 50% <output-file>
Or use built-in GUI programe 'gthumb'
(install with sudo aptitude install gthumb)
for some basic image manipulations, such as crop, rotate, resize, convert format
(for example, convert jpg image to png).
Click "Edit file" button on the right-hand side of toolbar, then select "Crop".
Add Lock Wallpaper to i3lock:
First test i3lock with i3lock -i /home/chad/.i3/wallpaper.png.
If succeeds, add the following line into ~/.i3/config:
bindsym $sup+l exec "i3lock -i /home/chad/.i3/wallpaper.png"