Check out example codes for "calendar.js:156 Uncaught TypeError: Cannot set property 'src' of null at HTMLDivElement.<anonymous>". It will help you in understanding the concepts better.
Code Example 1
If your img element looks like this, <img id="idToTarget" src=""> and SRC is set to a valid resource URI, then document.getElementById('idToTarget').src = SRC is how that line would be written.
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