The images added in full width header are tested with images in 16:9 format, minim 1920px width size. These images are set in CSS to be used in full width and full height. We noticed that the full height stretches the images in certain resolutions, and for the next update we’re looking to see if there is a better solution.
Small Slider – resizes images based on width and height: 480 x 270 (with sidebar), and 720 x 405 without sidebar. The images are cropped if, when resized, the height is larger than 270 or 405.
Infobox images are 670px max width, so your images, in general, should be at least 670px wide, even if you have set the small slider.