You should never change files in the plugins directory of WordPress, because you’ll lose the changes once you update the plugin!
Better approach is:
1) Copy this file /wp-content/plugins/yith-woocommerce-social-login/templates/social-icons.php
2) Paste it in /wp-content/themes/!!!YOUR_THEME_NAME_HERE!!!/theme/templates/
Then open the newly created file and locate the code around line 21st
'image_url' => YITH_YWSL_ASSETS_URL . '/images/' . $key . '.png',
this YITH_YWSL_ASSETS_URL is a constant that points to the assets directory in the plugin /plugins/yith-woocommerce-social-login/assets
but you can replace it with get_template_directory_uri() and you’ll end up with:
get_template_directory_uri() . '/images/' . $key . '.png'
The final step is to create and upload your images
/wp-content/themes/!!!YOUR_THEME_NAME_HERE!!!/images/facebook.png
/wp-content/themes/!!!YOUR_THEME_NAME_HERE!!!/images/google.png
/wp-content/themes/!!!YOUR_THEME_NAME_HERE!!!/images/twitter.png
That’s it!