Why the image is not responsive using AMP?

Question

I'm including the following script in my index.html:

<script src="https://cdn.ampproject.org/v0.js" type="text/javascript"></script>

After, in my body i have image tag with this strucure:

<amp-img
        src="//d9hhrg4mnvzow.cloudfront.net/welcome.jolyy.com/transparent.gif"                        

        srcset="//d9hhrg4mnvzow.cloudfront.net/welcome.jolyy.com/transparent.gif"
                        layout="responsive">
</amp-img>

but the image isn't responsive at all. Why ?