Originally Posted by Synergi
I checked the content file like someone said and got rid of the 1 on cover in the two spots. Still will not show up.
The 'id="cover"' trick has worked for me every time. (In fact, for books that don't have a cover at all, I drop the jpeg in there, and add a line like '<item href="cover.jpg" id="cover" media-type="image/jpeg"/>' to the manifest of the opf.)
I'm not sure what you are doing differently that would cause it to not work. Two suggestions are refresh the list (use the drop-down in the upper right to change the sort order ... someone else claimed this worked for them), and, obviously, reboot your nook color.
The jpeg thumbnail issue with Calibre will likely be addressed soon.