How to make full width images 'shorter'

I like to use the full width image block for a 'banner' which indicates a knowledge check is below it. When using other image blocks I have learnt I'm able to adjust the height of the uploaded image by using a no-fill, no-outline skinny horizontal rectangle centred on the image. The longer the rectangle, the shorter (less height) the image becomes.

This isn't working for full width blocks. In this instance I actually want the 'invisible' rectangle to be visible and solid colour. No matter how long I make that rectangle, the height of the uploaded image doesn't change. I want it about half the height shown in the screenshot, is this possible??