<div class="my_img"><img src="/img_01.JPG" alt=""></div>

 стили :
.my_img img{width: 10%; height: 10%;}

<div id="my_i"></div>

стили :
#my_i img{width: 100%; height: 100%;}

(кратко)

 1.
 document.getElementById('my_i').innerHTML = document.getElementsByClassName('my_img')[...].innerHTML;

 2.
 var img2 = document.createElement('img');
 img2.src = '/img_02.JPG';
 img2.alt = '';
 document.getElementById('my_i').removeChild(document.getElementById('my_i').childNodes[0]);
 document.getElementById('my_i').appendChild(img2);