The 'Full Width Image' element makes it possible to add an image, that is not restricted to the max width of images on the website. The element can be selected below the tab 'Content elements' when creating a new element. (1.11.1)
In the 'General' tab you can see that the content element type 'Full Width Image' is selected (1).
The element uses the regular header field (2), that is not visible in frontend (1.11.2)
To choose a picture you need to move on to the 'Image' tab. To choose a picture from your filelist click the add image button and select a picture. (1.11.3)