能做的也就是缓存
相关参考：
1.http://www.7dot9.com/2010/08/android-bitmap%E5%86%85%E5%AD%98%E9%99%90%E5%88%B6/
2.http://www.80hou.org.cn/articles/2010/09/21143520.html
3.http://ck19860613.iteye.com/blog/842732
