I have the following in one epub3 and it seems to work with epub3 aware applications:
Code:
<item id="cover_id" href="Images/cover.jpeg" media-type="image/jpeg" properties="cover-image"/>
Note that this is an epub3 only construct and Moonreader as an example does not handle epub3. As for your Kindle? Amazon has been changing how covers are handled on a regular basis. See the number of complaints about Send to Kindle (email, web page, whatever) having issues with covers doing now you see me, now you don't.
Edit: Also you can't use role="doc-cover" in an .opf file. Epubcheck will flag it as an error.