If you want this:
Do this:
Add an image block and an image;
Click on “Properties”;
Click on “Advanced formatting” on the right;
Adjust left and right margins to -50 (or a bigger/smaller number if -50 doesn’t produce the desired effect).
You can stretch the image in the header or footer the same way:
Double-click in the header/footer area;
Drag and drop the image block;
Access advanced formatting through properties;
Adjust left and right margins.
Alternatively, you can use the Cover page to easily stretch the image to the page width:
Add a Cover page by clicking "+" below or under a page > Cover page;
Drag and drop the image block;
Add your image;
Drag the image by the blue dots at the corners to stretch it.
