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开发者_JAVA技巧 size. Please suggest how to do it and achieve this with an ease.

Thanks in advance.

how to insert an image into a post , YouTube tutorial

http://www.youtube.com/watch?v=c--FXmLJ59A