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Optimal? That's not really a question with an answer. Optimal for what?
Unless you intend to produce covers that are tailored to a particular device (which I would discourage), a sufficiently large number of pixels, and a sufficiently small amount of compression.
IMO, over 2,000 pixels high and a moderate JPEG compression would be optimal. It will look good on anything. (Although low contrast colour images can look poor on greyscale devices no matter what their resolution.)
JPEG for the file format. It usually gives the best compromise between size and number of pixels, so long as you don't set the compression too high which can introduce obvious artifacts.
If, however, you are generating the image yourself, the original ought to always be saved as TIFF or PNG (lossless).
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