<img src="/image-in-root.jpg" alt="image in same dir as index.html"/>
or
<img src="./image-in-root.jpg" alt="image in same dir as index.html"/>
or
<img src="../../image-2-dirs-up.jpg" alt="image 2 dirs up from this file position"/>
or
<img src="/images/image-in-image-dir.jpg" alt="image in /image dir from root"/>
or
<img src="./images/image-in-image-dir.jpg" alt="image in /image dir one level below current"/>