Twenty Fourteen uses 2 custom media sizes:
- 672x372px for post thumbnail image size
- and 1038x576px for ‘twentyfourteen-full-width’ custom image size
So for images uploaded after the theme is activated, these 2 size are created automatically, but for images uploaded before twentyfourteen is activated, an available size is used and image is resized via html/css properties.
What I suggest is for new images upload files having width >= 1038 px and height >= 576 px and let theme resize them.
For images already uploaded a plugin like Regenerate Thumbnails can be very usefull (but images must be larger than custom size required by theme or the thumbnail will not be generated).