You can try doing
<p>
<a href="https://stackoverflow.com/questions/15458650/MY WEBSITE LINK" target="_blank">
<img src="https://stackoverflow.com/questions/15458650/IMAGE LINK" style="width:100%;" border="0" alt="Null">
</a>
</p>
This should scale your image if in a fluid layout.
For responsive (meaning your layout reacts to the size of the window) you can add a class to the image and use @media
queries in CSS to change the width of the image.
Note that changing the height of the image will mess with the ratio.