[Updated December 2015 to include Twenty Sixteen.]
Any sufficiently large system is bound to give rise to inconsistencies, which may be why I always find myself looking up the recommended size for header images, depending on the WordPress theme being used for the project at hand. It’s difficult to believe there’s not a better way of handling header images, but until there is, here’s a handy reference table.
|Theme||Width px||Height px||Ratio|
* The fine print in TwentyTwelve says something to the effect that smaller images are acceptable, and that the maximum image width is 2,000 px.
If I remember correctly, Kubrick did not have the option of a header image, but at any rate you can no longer download it from the WordPress theme library. If you’ve got Kubrick and you’re still using it, you probably know by now what it needs.