Excuse me, but I really don’t understand-
The image you talk about is the "AID"-Logo with its red lines.
And this image shall span – what? The width of the browser window or the page width?
If browser wide: Place a rectangle, and set it to 100% width. Then click onto the word "Fill" (= "Fläche") to the left in the upper command strip. There you can choose an image. Set this image to "Scale to Fill", (= "Füllend skalieren") or whatever you like.