Here are the steps to make a header image span the width without being cut off:
In the admin menu click the +Content button and choose Objects then the image object
In that new block that was added, click the "Choose an image" button and upload/select the image you want
In the block, click the gear icon in the upper left for the block settings
Uncheck the "Default Block Spacing" setting then change the top and bottom padding to zero
Uncheck the "Boxed in Container" setting
Click the "Save Changes" button
In the block, click the gear icon in the upper left for the block settings
Click the HTML button in the upper left. Now for the scary part :-)
Look for something like the following: <img class="img-fluid" alt="" title="" src="data/files/yourfilename.png">
Change class="img-fluid" to class="img-fluid w-100"
Click "Save changes" at the bottom
That's it! The image should not span the width without being cut off