Change image size via parent div

Apply 100% width and height to your image:

<div style="height:42px;width:42px">
  <img src="http://someimage.jpg" style="width:100%; height:100%">
</div>

This way it will same size of its parent.

I'm not sure about what you mean by "I have no access to image" But if you have access to parent div you can do the following:

Firs give id or class to your div:

<div class="parent">
   <img src="http://someimage.jpg">
</div>

Than add this to your css:

.parent {
   width: 42px; /* I took the width from your post and placed it in css */
   height: 42px;
}

/* This will style any <img> element in .parent div */
.parent img {
   height: 100%;
   width: 100%;
}