Is there any option to crop after choosing image using filepicker? In WP8, photochoser task had a similar option, is there any alternative to that for WP8.1?
The file picker returns the file and doesn't do any direct manipulations. You can crop when you read the file in by passing a BitmapTransform with cropping information into your BitmapDecoder.GetPixelDataAsync call. The How to crop bitmap in a Windows Store app (C#) sample demonstrates this - it targets Windows Store apps rather than Windows Phone, but the cropping code will be the same for both Windows Runtime targets.
There are also several external libraries which can provide cropping behaviour, such as the Lumia Imaging SDK and WriteableBitmapEx)
If you want to do this visually you'll need to provide your own UI for that. The How to crop bitmap in a Windows Store app (C#) sample demonstrates this as well, but will need some changes to fit a phone's screen.