In order to avoid any adverse affect on the other custom images you can  include the CSS styling code in the HTML section of your blog post.

image correction

Step 1: Access the HTML editor of blog post.
 
Step 2: Add your styling code manually for the selected text area.

  Find the chunk of text in the HTML view, and then include your styling code. 
 
just paste/type the following css code at the bottom of your post and you are done .

This will rectify all the images present in your current post:
 
<!--css code for image correction-->
<style>

.post-body img {
width: 100%;
height: 100%;
display: block;
}

</style>