Amazon says having a cover image and an .xhtml cover page is "bad form, old boy." I tend to agree...
Firstly, if you DO go to the cover image (from the menu) it nicely fills the screen (or should if it's 600x800), then the next page you get a picture that poorly fills the screen - and opening the ebook on the cover.xhtml also means you have to push "Back" twice to actually go back one page to the cover proper. Not to mention when the book opens, you get the title bar at the top borking things up even more.
That said... here's how you do it.
For the cover image, right click and "Add Semantics - Cover". That'll create the entry in the erm meta-data file. Do the same for the cover.xhtml file - another entry created.
Can't recall the name of that file right now
(down the bottom of the book browser, not the toc.ncx file) but open that puppy and down the bottom will be the "semantic" data.
Can't remember the details right now
but there'll be something like "type=cover" and "something=Cover" ... copy that whole line and paste it again.
Change both the type and the something to start and... bingo. Once converted, the .mobi will (should!) open to cover.xhtml.
When I'm on a computer that works properly, I can put the exact info in... remind me later :lol: