First option

First you need to install jpegoptim on the server

sudo apt install optipng

Then set up automatic image optimization

#[min] (0-59)
#|   [hour] (0-23)
#|   |    [day of the month] (1-31)
#|   |    |    [month] (1-12)
#|   |    |    |    [days of the week] (0-6 with 0=Sun)
#|   |    |    |    |    [user](only for system cron)
#|   |    |    |    |    |    [command]
#|   |    |    |    |    |    |
0    3    *    *    *         /usr/bin/find /var/www/sites/default/files/ -iname '*.png' -print0 |xargs -0 optipng -o7
#EOF
Second option
sudo apt install pngcrush

Best compression

pngcrush -rem alla -rem text -reduce -brute in.png out.png

Automatic optimization

pngcrush -reduce -brute in.png out.png

Remove all color correction data from file

pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB in.png out.png

Remove auxiliary data

pngcrush -rem alla -rem text in.png out.png

Optimize all files in a loop

for file in *.png ; do pngcrush -reduce -brute -rem alla -rem gAMA -rem cHRM -rem iCCP -rem sRGB "$file" "${file%.png}-crushed.png" && mv "${file%.png}-crushed.png" "$file" ; done

First you need to install jpegoptim on the server

sudo apt install jpegoptim

Then set up automatic image optimization

#[min] (0-59)
#|   [hour] (0-23)
#|   |    [day of the month] (1-31)
#|   |    |    [month] (1-12)
#|   |    |    |    [days of the week] (0-6 with 0=Sun)
#|   |    |    |    |    [user](only for system cron)
#|   |    |    |    |    |    [command]
#|   |    |    |    |    |    |
0    3    *    *    *         /usr/local/bin/jpegoptim --strip-all /usr/local/www/sites/default/files/*.jpg
#EOF