I have commented this line in the file multizoom.js of the module website_multi_image_zoom for Odoo 8.

//$imgObj.attr('src', this.src).css({width: w || options.width || this.width, height: (h = +(h || options.height || this.height))});

This line set a fixed height and width for the main image on load.
Now, images are not stretched.
Thanks all