<img srcset="
  /red_bycicle__high.jpg 4000w,
  /red_bycicle__med.jpg 2000w,
  /red_bycicle__low.jpg 800w,
" 
  sizes="(max-width: 800px) 700px, 1000px"
  alt="Red bycicle wheel"
  src="/red_bycicle__med.jpg"
/>