There are two fixed sizes for images on Sutori. You can have an image displayed on one side of the timeline or expand it so that it covers the entire width by pressing on the button as on the screenshot below.

Please note that the display of a story adapts to the size of your screen. If you do not see the arrows icon like above, you either have to expand the size of the window or your screen is too small (tablets and mobile phone screens for example).