@ruchin78
use {{ doc.image }} as path if your file is in same doctype like in Item Doctype we have Item image.
If you want to use another image, you can upload anywhere and give file path like @priya_s said.
e.g.
<img src="http://sbkolate.com/wp-content/uploads/2015/09/sbk_logo-Copy.png" height="35%" width="85%">