Android 美工工具，以及图片压缩
(1)TinyPNG（压缩图片资源利器）https://tinypng.com/
Original transparent PNG
File size: 57 KB vs 
Shrunk transparent PNG
File size: 15 KB
(2)一个PhotoShop插件，可以生成点击状态的，普通状态，方便开发动作。
（3）设计师也需要有一套标准来设计Android UI，所有的Android UI设计指南。
http://developer.android.com/design/style/devices-displays.html
Be flexible
Stretch and compress your layouts to accommodate various heights and widths.
Optimize layouts
On larger devices, take advantage of extra screen real estate. Create compound views that combine multiple views to reveal more content and ease navigation.
Assets for all
Provide resources for different screen densities (DPI) to ensure that your app looks great on any device.
