This plugin from WPZA provides your website with a shortcode to display AMP images, rather than using the standard HTML img tags in WordPress.
To insert the image, you must use the image attribute as such: [amp-img image="https://wpza.net/example-image.jpg"]
To insert the alt text, you must use the alt attribute as such: [amp-img image="https://wpza.net/example-image.jpg" alt="Example image alt text"]