how to set image tag source from web.config?

how to set image tag source from web.config?

How to set image tag source from reading the key from AppSettings section
in web.config in html file.
I am using one html file, in which there is an image tag and I want to set
image source from web.config file key.
The web.config key is: <add key="Image" value="Image.gif" />
But i am not able to set the key on source.
I have tried this :
<img src="<%= ConfigurationSettings.AppSettings["Image"] %>" width="685"
height="177" />
but is not reading the source from there.