Yes, This issue is due to cached image, but can't change the image name as it is generating dynamically from database.
Thanks a lot for ozkar to direct me in right direction. :)
Now to resolve this issue I am adding a query string with image url. I had to replace only one line in my code & this is working fine now. :)
parentDiv.Style.Add("background-image", "url('" + strImagePath + "')");
replaced by
parentDiv.Style.Add("background-image", "url('" + strImagePath + "?x=" + imageId + "')");
Where imageId is unique in my application. Any unique value can be used at the place of imageId.
Thanks
Prem Kishor