When you create static methods in your form code, the source code looks like this

public int imageNum=0;

。。。。。。

 public static int button1_clicknum()
        {
            return imageNum;
        }

This is followed by an error such as the following. The error is caused by the fact that the static method cannot directly access the non-static member, and the non-static field needs to be referenced, which can be a private member of an instance. To correct this, change imagenum to a static member.
Public static int imageNum = 0;