Use {!$Resource} to reference an existing static resource. The format is {!$Resource.nameOfResource}, such as {!$Resource.TestImage}.
<apex:image url="{!$Resource.TestImage}" width="50" height="50"/>
<apex:image url="{!URLFOR($Resource.TestZip, 'images/Bluehills.jpg')}" width="50" height="50"/>
You can also use dynamic references to reference static resources. For example, {!$Resource[appLogo]}, assuming there is an appLogo property or getAppLogo() method in your page’s controller.