//Go to the file location of .ashx file
//Open with your favourate editor
//you may see
//<%@ WebHandler Language="C#" CodeBehind="ClassName" Class="ClassName" %>
<%@ WebHandler Language="C#" CodeBehind="img.ashx.cs" Class="MyApplication.Core.img" %>
//Replace 
//<%@ WebHandler Language="C#" CodeBehind="ClassName" Class="ClassName" %>
//with
//<%@ WebHandler Language="C#" CodeBehind="ClassName" Class="Namespace.ClassName" %>
<%@ WebHandler Language="C#" CodeBehind="img.ashx.cs" Class="MyApplication.Core.Handler.img" %>