Hi Senthil,
The background image is controlled by the form rendering template given in the properties of the survey like here :

So you need to navigate to this template here under Administration>configuration>Form rendering. Here, click on page layout and change the image . Upon publishing the survey , you should get the new image updated.

For my example this path is present in the server <directory>\datakit\xtk\fra\img\report folder and the image is present in the xtk:images schema.
Regards,
Adhiyan