Hello Ann,
Have you tried storing the image url in a token and then just reference the token within the image tag inside the mktEditable region? The image should get rendered based on the section's style settings regardless of its actual dimensions.
Not sure if this answers your question, but I've found it makes things much easier if I store the image url in a token rather than manually going into the mktEditable region and replace the image. I know it will always get rendered properly regardless of the image's dimensions.
Here's an example:
<div class="mktEditable" id="header-image" ><img style="display: block;" src="{{my.HeaderImage}}" alt="" height="230" width="560" border="0"></div>
Hope this helps!