I've a folder titled 'html_playground' inside folder 'www' which is my web root directory of Wampserver.
Inside the directory 'html_palyground' I have a a file titled 'demo.html' which contains following HTML code :
<img src="/images/html5.gif" alt="HTML5 Icon" style="width:128px;height:128px;">
<img src="images/html5.gif" alt="HTML5 Icon" style="width:128px;height:128px;">
The forward slash at the beginning instructs the browser to resolve the path relative to your web root directory on http://localhost/
As you mentioned in your comment this would correspond to the www/ directory on your file system. So
/html_playground/images/html5.gif should work
Basically the forward slash is just appending the specified path to the domain.