Examples abound on the web for CSS responsive image so we tried one randomly chosen example in epub3x environ for potential use as the cover page image. My CSS refinements seemed viable [at the time] and passed certain epub3 ereaders, but failed on epub2 (+ had other code hurdles).
EDIT: Rather than have heaps of superfluous words in my comments....we'll trim this down a bit.
1. Use SVG cover (as 'RbnJrg' suggests).
2. Styled <div> for cover (as 'JSWolf' suggests).
Both commenters have viable cover solutions; either cover method is good.
3. Experimental link below.
Live JSFiddle:
Link
My JSFiddle Epub3x CSS code test is for my needs (for Epub3x only, with a focus on Android cell phone based users - my primary user base).