Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

Yes, you can override this same image file name into your current template, this is the override path :

JoomlaFolder/templates/YourCurrentTemplate/html/com_easydiscuss/images/placeholder-facebook.png

Once you created this, you have to re-scrape again your discussion post URL from the Facebook debugger tool https://developers.facebook.com/tools/debug/ so Facebook can able to re-crawl again with your new placeholder image.