So, after manually resizing and saving several icons of different sizes, I’m bored with this chore. I did some search on google, and I fond out there are indeed better ways — let program do it for you.
You can of course write your own program to do so, but if you just want a quick solution, here are two websites for you:
App Icon Sizes: http://www.appiconsizes.com/
For best outcome, upload an image of size 1024 points.
These two are kind of similar, but I find any single one of them not enough for the numerous sizes required for app icons. You may probably use them both and match the right sizes with the correct icons.