그냥 max-width:100% 만 추가하면 가로세로 비율이 안맞는 경우 아래와 같이 추가한다.

/* Images */
.entry-content img,
.post-content img,
.comment-content img,
.widget img {
max-width: 97.5%; /* Fluid images for posts, comments, and widgets */
}
img[class*="align"],
img[class*="wp-image-"] {
height: auto; /* Make sure images with WordPress-added height and width attributes are scaled correctly */
}
 
img.size-full {
max-width: 97.5%;
width: auto; /* Prevent stretching of full-size images with height and width attributes in IE8 */
}