You mean Image of template it self or background image set through code for a template ?
for first case you need to change <Path>/templates/<Your template>/thumbnail.png
for second case just change code and point it to new background image.
Thank you very much!!!
Basically, I need to change the fixed back image in website (http://www.hanwha.com >> you will see the orange background image, and i need to change this to new image. ) Could you tell me more details how it would be possible to do this task?
Make a friend with firebug & you could find yourself from where it is getting applied.
For the website you have given the background image it is using is  and it is getting set in css of body tag . So Find the css at  and modify it OR replace  with other image with same file name.
background: url("images/bg-body.jpg") repeat-x scroll 50% 0 #FFFFFF;
font: 14px/18px Arial,Verdana,Helvetica,sans-serif;