How you can do, if i wish to take away the default image from feed and assign another image

you'll have to replace the look in your server. The simplest method of doing this really is to (while using the Opera) right click the image and click on "Copy Image Location" then past it inside a new internet browser and you'll have the road to your email. You may also check it out in IE by Right Click after which qualities. One other way to do this is by using a Joomla! extension which will help you to modify it.

Hope it will help!