Hi Imran,
For Facebook, you can try changing the image here JoomlaFolder/media/com_easydiscuss/images/default_facebook.png
Remember to backup the original file in case you want to revert the changes. Also, make sure the image dimensions, format and size is similar to the default image.
Then try to create new autoposting posts and see if the image uses your new image file.

For Twitter, unfortunately there is no way to change the image. If you check the image source from the browser console, you can see that the image in Twitter's autopost is rendered by Twitter itself.

Can we disable the Pic feature itself ?
I mean when ever we post it goes without a pic only plain text is posted on FB and twitter ?

It is not possible for that currently Imran.