Full width image on mobile

Hi, I have an image that I need to be full width on mobile. That’s 100% width of whatever the mobile screen size is edge to edge.

Please note that this is different to the stretch background feature. I want it to apply to an image or a box element not a section.

Width:100%
Height:auto;

I’ve tried adding some CSS but I may be doing something wrong. Is there a script alternative perhaps?

Many thanks,
Habib