CB Gallery: Rotation of pictures

This is just for my information. Many photos uploaded from smartphone are not in the proper orientation and need to be rotated by 90° or 180° but they can't anticipate from their smartphone so, it is only while displaying the photo in gallery that we see the reeel orientation. However, it is not possible to change this orientation. 

For a dvelopper, what should be the difficulty to add such feature ( a button to rotate image by 90%) ?

EXIF orientation is already respected. In both client side and server side uploads. So if they took their photo with a mobile device and uploaded it should orientate correctly, but from a previous post I recall you turned off always resampling so some images probably aren't being resampled and thus this behavior can possibly be bypassed.

Aside from that what you're asking for already exists. Open a photo then view the dropdown menu for it where you'll see menu options to rotate a photo left or right and you can do so as many times as you need.