An Item in a Jar/Zip-file is not really a File but a resource. A resource is found using a URL. When asking the Jar-file to get the resource that is found in file:/path/to/jarfile/my.jar!/img/abc.png you are not really getting a file but a handle to where to start reading.
If you look at the ClassLoader-class (that should be used, not a specific class) you find that the method getResource(
String s) returns a URL that can then be used to get the resource. (
http://download.oracle.com/javase/6/docs/api/java/lang/ClassLoader.html#getResource%28java.lang.String%29)
For images, you can actually use the Toolkit-class.