Each preview of a Picture document type needs to be as wide as possible. Generate a thumbnail is useful to display light weight images, but not necessary for the width and height.
To always have an image that fit the screen size, the .fullWidth class (css rule: width: 100%) has to be added each time you call an image.
That way, no matter which screen resolution you have, the image will take the width of its container.
Huge benefit: it also avoid an overflow of a big image from its small container.