If your favorite tool is "shutter" you can try to create a little plugin for it.
Creating Plugin for Shutter.
1) Make sure you have installed optipng, or install it with:
sudo apt-get install optipng
2) If shutter is running close it or kill it..
3) Create a folder for the plugin and give it the correct perms. (eg:optipngplugin)
4) Create the script (eg:optipngplugin)
gksudo gedit /usr/share/shutter/resources/system/plugins/shell/optipngplugin/optipngplugin
5) Put this content in the script:
#!/usr/bin/env bash
TEXTDOMAIN=shutter-plugins
TEXTDOMAINDIR=$SHUTTER_INTL
PLUGIN_NAME=$"OptiPNG Plugin"
PLUGIN_SORT=$"Recompress"
PLUGIN_TIP=$"OptiPNG is a PNG optimizer that recompresses image files to a smaller size"
PLUGIN_EXT="image/png"
if [[ "${1}" = "name" ]];then
echo "${PLUGIN_NAME}"
exit 0
elif [[ "${1}" = "sort" ]];then
echo "${PLUGIN_SORT}"
exit 0
elif [[ "${1}" = "tip" ]];then
echo "${PLUGIN_TIP}"
exit 0
elif [[ "${1}" = "ext" ]];then
echo "${PLUGIN_EXT}"
exit 0
fi
FILE="${1}"
#LOGO="/usr/share/shutter/resources/system/plugins/shell/optipngplugin/optipngplugin.png"
optipng -o7 "${FILE}"
exit 0
In my case I choose the optimization level of OptiPNG to Maximun (-o7) (very slow), and I added an image as a logo.
Man Pages

NOTE: Feel free to change the script according to your needs
6) Save the changes and give it perms.
sudo chmod 755 /usr/share/shutter/resources/system/plugins/shell/optipngplugin/optipngplugin
7) In my case I edited a png logo with gimp, and I saved as "optipngplugin.png" in my desktop.
8) copy the logo to the plugin directory (give it the same name as the plugin plus .png
) and the correct permissions:
sudo cp /home/user/Desktop/optipngplugin.png
/usr/share/shutter/resources/system/plugins/shell/optipngplugin/optipngplugin.png
sudo chmod 644
/usr/share/shutter/resources/system/plugins/shell/optipngplugin/optipngplugin.png
9) Test the plugin. Open shutter and take a screenshot of your desktop.

Right click in the screenshot and click in "Run a Plugin...", then choose "OptiPNG Plugin" from the list.

10) Run the plugin to recompress the screenshot you took.

NOTE: If you have to add something to the script or some change. you can run shutter with this command to force it to reload all plugins
Hope its help.