Method 1:

The CSS

Add the following code to your CSS file. That will make the images scalable according to screen size.

Removing automatic height and width in WordPress tags

Now drag the window to see the image scaling in action. You’ll notice that the images in your WordPress blog scale weirdly. They horizontal scaling fine but vertical scaling in WordPress images are all wrong.
To make the images resizable proportionately in WordPress, We have to remove the automatic width and height values WordPress add for < img > tags.

As an example, We have to change this:

To This:

For the images that are in a post or a static page/template page, all you have to do is, add the above CSS to the style.css file, and then remove the ‘width’ and ‘height’ properties from the < img > tag in your WordPress editor. That’s it!
But for the image that are displayed dynamically by WordPress, such as post thumbnails, the width and height needs to be removed dynamically using a function.

Add the following function to your functions.php file.

Then when you call for those post thumbnail images in you template.php page, replace,

With This:

That’s it. Drag and resize the browser to see your responsive WordPress images in action! Here’s a demo.

Method 2:

The above will not work for some themes.
If you are one of the few that  it did not work, you can still get your image issue solved using below function.

Add the following function to your functions.php file.

This removes inline width and height attributes from images retrieved with 

,
and prevents those attributes from being added to new images added to the editor.

add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );  
add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 ); 
function remove_thumbnail_dimensions( $html ) {     
$html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );     
return $html; }