In your functions.php
file, you need to define $content_width
, which will cause WordPress to scale large-size images, and also embedded videos, accordingly.
e.g. if the width of your content area is 600px:
if ( ! isset( $content_width ) ) {
$content_width = 600;
}
You will probably also want to add accompanying CSS, e.g.:
#content img {
width: auto;
max-width: 600px;
height: auto;
}
That way, when the image is scaled, it will maintain its proportion; rather than merely having its width reduced, the height will also be scaled proportionately.