I don't think there's a general solution. It depends on how the books is coded (and also Prince's behaviour has changed between versions in the past).
In many of my books I have something like:
Code:
/* specific code for the cover image */
@page cover {
size: 600px 800px; /* hard-coded cover image size */
margin: 0 -100px; /* make virtual page width 800px */
}
body.cover {
page: cover;
}
But this relies on:
* The cover image being in a <body class="cover"> tag.
* The cover image being 600x800 px exactly.
* The cover image being coded to use the full page.
At least at some point this worked fine for me.