how can put a javascript variable in img tag

how can put a javascript variable in img tag:
Image Url : "https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgztk1AeqLqK7P8XBCHAYEHcmN9CAXR-pQv-hWAzmn40L20LA0K43MyPHyDuWTMbYimVdjZ23XDWxP9g2y7_LHxyz97UcYBnag0VGWeI0YOuRKFLtD6RmNwfL8C6k4sJVoSuv2ibTYPQc8/s300/1.jpg"

<img id="myImg" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgztk1AeqLqK7P8XBCHAYEHcmN9CAXR-pQv-hWAzmn40L20LA0K43MyPHyDuWTMbYimVdjZ23XDWxP9g2y7_LHxyz97UcYBnag0VGWeI0YOuRKFLtD6RmNwfL8C6k4sJVoSuv2ibTYPQc8/s300/1.jpg" width="350" height="500"> <button onclick="myFunction()">Next</button> <script> var a ="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgCqKLRaIhLmloc7Hj_xMkZgKVofFj7kduYVbGzwyBTFIyhQpqoppQl0oFD8yYlW0tBi8VNhkRtl7KDBzmsKKO0zwC0OP4v9vJ90xlMNEyT6kvWEdalH68EMDmxt7AMRXeYq-rp-Iioq9Y/s1600/Lakshmi+Rai+hot+stills+(25).jpg"; function myFunction() { document.getElementById("myImg").src = a; } </script>

Check Output here: