.
Could someone please, explain why this thread (it used to be Sticky) is now under Library Management? 
As to the question:
Copy your image somewhere, let’s say on:
C:/Program Files/Calibre2/resources/images/old_paper.jpg
Assuming your image is called “old_paper.jpg”
Open an ebook; on the reader preferences paste the following :
body {
background-image:url("file:///C:/Program Files/Calibre2/resources/images/old_paper.png");
}
Close the preferences dialog.
Enjoy