get src by id

for dyanmic images display image on popup 

 function showImage(i) {

  

   

        var img = document.getElementById('img_'+i);

        img.className = 'element';

        var modalImg = document.getElementById("img01");

        var captionText = document.getElementById("caption");

        modal.style.display = "block";

        modalImg.src = img.currentSrc;

        captionText.innerHTML = this.alt;

    }

read this link to know how to access the subnode of each element

for example get src.