Maybe not exactly what you were asking about, but I keep a folder for alternate cover versions, back covers and miscellaneous graphics and pictures related to my ebooks, and have made a link to the pic in the Comments field:
(under the HTML Source tab)
Code:
<hr>
<h2 align="center">Alternate Cover</h2>
<p align="center"><a href="file:///D:/eBooks/Temp/Covers/Cover Name.jpg"><img src="file:///D:/eBooks/Temp/Covers/Cover Name.jpg" width="150"></a></p>
If there are spaces in the filename, Calibre converts them into "%20" so it will look strange if you edit it later.
I kept the width of the thumbnail at 150 pixels to fit the narrow Details strip, but it links to the full-size picture when clicked on.