How to crop pictures into the size of an iphone 4s?

I want to specifically crop my photos into all iphone 4s photos but i don't know the exact way to do that haha i've tried but please do help me

IPhone 4s pictures are 640x960 (screen capture PNG) or 2448x3264 (camera JPG)

There's a very useful free tool that allows you to most anything with photos and it's free… IRFANVIEW. Lots of features, including the ability to crop, re-size, touch-up, add effects and convert your pictures into PNG format for your I-Phone.
It's an excellent tool… You will love it
http://www.irfanview.com/