Image not shown in a portlet

I created a portlet and added this line to a JSP. However, the image icon.png is not shown.

<img src="<%=renderResponse.encodeURL(renderRequest.getContextPath() + "/icon.png") %>">

I also tried this line, but it did not work either.

<img src="<%=renderResponse.encodeURL("/liferay" + renderRequest.getContextPath() + "/icon.png") %>">

I am using Liferay 6.1.1-0 on Ubuntu Linux 12.04 LTS 64-bit.

Please help. What command am I missing?