Taking picture using front camera of s7,the image on screen side is bit distorted.Help me?

Your device is Not faulty. The front camera of Samsung Galaxy S7 uses wide-angle lens. This is very useful and convenient because it’s suitable for taking wide-angle photos when using the Camera close to the Subject. (e.g. when taking a picture of yourself.) Minor image distortion may occur in wide-angle photos especially at the edges of the preview screen due to the properties of the lens used to take a wider screen image. 

It does not affect device performance problems. At that time, enable Shape correction option to correct your face shape. Shape correction option corrects your face distorted at the edges dynamically according to angles and distance from center. You can set the option on Beauty effects. Please note after shape correction, the background may appear slightly distorted since the correction algorithm is applied to overall image. If you do not want this, please use after deactivating the Shape correction. 
 
IMPORTANT :  After shape correction, the background may appear slightly distorted since the correction algorithm is applied to overall image. If you do not want this, please use after deactivating the Shape correction.