<meta name="viewport" content="width=device-width, user-scalable=no"/>
<style>
#img1{
   content:url(https://www.gravatar.com/avatar/713e5befd94e05e3d05d2a0be83a55d0);
}
@media screen and (max-width: 768px){
   #img1{
      content:url(https://www.gravatar.com/avatar/713e5befd94e05e3d05d2a0be83a55d1);
   }
}
</style>
<img id=img1>