var str="http://www.baidu.com/Sorado/pic/L21udC9jeXkvYXBhY2hlLXRvbWNhdC04LjAuMjYvYmluL0lNRy8yMDE2LzA4MDMvMTQ3MDIxMDY5MTEyNy5naWY="
var len=str.split("/");
icon= str.pop();

方案二:

var str="http://www.baidu.com/Sorado/pic/L21udC9jeXkvYXBhY2hlLXRvbWNhdC04LjAuMjYvYmluL0lNRy8yMDE2LzA4MDMvMTQ3MDIxMDY5MTEyNy5naWY="  

var a = str.match(/\/{1}\w+=$/)[0].slice(1);

a
"L21udC9jeXkvYXBhY2hlLXRvbWNhdC04LjAuMjYvYmluL0lNRy8yMDE2LzA4MDMvMTQ3MDIxMDY5MTEyNy5naWY="