The visible part of the cover photo view on the listing is approx. a third - that in the center of the picture.
I'm quite savvy with photoshop, so it wasn't a problem placing it on a neutral background in such a way that the middle part wasn't too bad. But it took multiple uploadings to check how it looked. If you aren't experienced in photo adaption, then it'll be a bit more difficult, unless you know somebody who is...
I also haven't been able to figure out the best picture size and dpi for the site, but have been getting the impression again and again that the site automatically adjusts (too) large pictures to fit, since even large 20MB photos are scrolled on the site as easily and quickly as 2MB ones.