You can use this superb plugin:
RetconHTML
Ever have a client put 4 MB PNG files in their Redactor fields, failing to apply any of your meticulously crafted image transforms? Writers consistently using the tag for headers? Have you ever needed to implement lazy loading of images embedded in WYSIWYG content, responsive images or wanted to remove pesky inline styles without breaking a sweat? This is for you.
It is super easy. Just add this filter to your WYSIWYG template.
{{ entry.wysiwyg | retcon('transform', 'yourImageTransform') }}
As an option you can install Imager.
If installed, Retcon uses Imager to apply the transform. Imager is faster and more flexibel than the standard image transforms in Craft.