I have the same problems. I'm not really sure how the cover.html works.
But I could make a png-graphic file to show as the "cover" and "thumbnail" of the book. I added a line to <manifest..> and the <spine.. > section of content.opf.
It looks like this:
<item id="logo" href="logo.png" media-type="image/png" />
<spine xmlns="http://www.idpf.org/2007/opf" toc="ncxtoc">
It's the line with the logo. Make sure, that it is at the very beginning of the sections. Make sure, that the file logo.png is in the same folder as content.opf.
I've tested it with ADE, cause I do not have a Sony PRS...