Thanks for your feedback!
Recommended header image size is 1018 pixels by 250 pixels but it doesn’t mean that you can’t use different size especially for height.
You can get this done by using WordPress built in gallery function like I have done it on theme demo: http://colorawesomeness.com/travelify/post-format-test-gallery/
More about this function you can read here: http://codex.wordpress.org/The_WordPress_Gallery
You can use various different gallery plugins to achieve similar result.