Responsive Floating Boxes

[valarcher]

Also, do you perhaps know the css for this?

Images for different screen widths

Hi - I need a box that contains a 728px banner on a computer screen, but displays as a 300px banner in mobile. How do I say to the browser - display this image if screen is 800px or wider, BUT display this image if screen is narrower than 800px? thanks! - Val

thanks! - Val