Create a class with required properties like Image Id,Image etc.Store the values in those prooperties and convert it into JSON using Newtonsoft Library.
You can serialize it into json object as follows.
ImageProperties image = new ImageProperties();
image.ImageId=101;
image.ImageData = "xxxxx";
JsonConvert.SerializeObject(image);
Let me know if you need any help.Following
link will help for you reference regarding the Json with NewtonSoft library.