How to insert a particular image in a WordPress post

I have theme which shows some default theme in it.The themes name is Flow Hub.In first page and all of the other pages I have a default image there.But my requirement is to replace that default image in there,with the image which I am inserting with the Post.
It could be thumbnail or any of the size.
Please suggest how to do it and achieve this with an ease.

Thanks in advance.