I would recommend using URI to create a File like this:
String url = "http://i2.kym-cdn.com/photos/images/original/000/748/132/b84.png";
URI uri = URI.create(url);
File f = new File(uri.getPath());
The uri.getPath() returns only the path portion of the url (i.e. removes the scheme, host, etc.) and produces this:
/photos/images/original/000/748/132/b84.png
You can then use the created File object to extract the file name from the full path:
String fileName = f.getName();
System.out.println(fileName);
Output of print statement would be:
b84.png
However if you are not at all concerned by the input format of the url(s) then the substring answers are more terse. I figured I would offer an alternative. Hope it helps.