Valid Reason for Using IDisposable intreface

[C# Programming]

Hi All,
Can anyone please clarify my dout about IDisposable intreface in C#.
We all know tat IDisposable interface is using for disposing unmanaged resources.
I have a class which contains following code. here i have implimented the Dispose method from IDisposable interface.

class ClassA:IDisposable
{
public ClassA()
{
Console.WriteLine("ClassBeingTested: Constructor");

}
private bool disposed = false;
Image img = null;



public Image Image
{

get { return img; }
}


~ClassA()
{
Console.WriteLine("ClassBeingTested: Destructor");
// call Dispose with false. Since we're in the
// destructor call, the managed resources will be
// disposed of anyways.
Dispose(false);

}

public void Dispose()
{
Console.WriteLine("ClassBeingTested: Dispose");
// dispose of the managed and unmanaged resources
Dispose(true);

// tell the GC that the Finalize process no longer needs
// to be run for this object.
GC.SuppressFinalize(this);
}

protected virtual void Dispose(bool disposeManagedResources)
{
// process only if mananged and unmanaged resources have
// not been disposed of.
if (!this.disposed)
{
Console.WriteLine("ClassBeingTested: Resources not disposed");
if (disposeManagedResources)
{
Console.WriteLine("ClassBeingTested: Disposing managed resources");
// dispose managed resources
if (img != null)
{
img.Dispose();
img = null;
}
}
// dispose unmanaged resources
Console.WriteLine("ClassBeingTested: Disposing unmanaged resouces");
disposed = true;
}
else
{
Console.WriteLine("ClassBeingTested: Resources already disposed");
}
}
// loading an image
public void LoadImage(string file)
{
Console.WriteLine("ClassBeingTested: LoadImage");
img = Image.FromFile(file);
}


}

What my doubt is why i need to impliment the Dispose method from IDisposable interface?. Instead of that i can create my own Dispose method in my class without inheriting from IDisposable interface which i have given below.

for the class below i haven't inherited my class from IDisposable interface. instead of that i created my own dispose method. this also works fine.
class ClassA
{
public ClassA()
{
Console.WriteLine("ClassBeingTested: Constructor");

}
private bool disposed = false;
Image img = null;



public Image Image
{

get { return img; }
}


~ClassA()
{
Console.WriteLine("ClassBeingTested: Destructor");
// call Dispose with false. Since we're in the
// destructor call, the managed resources will be
// disposed of anyways.
Dispose(false);

}

public void Dispose()
{
Console.WriteLine("ClassBeingTested: Dispose");
// dispose of the managed and unmanaged resources
Dispose(true);

// tell the GC that the Finalize process no longer needs
// to be run for this object.
GC.SuppressFinalize(this);
}

protected virtual void Dispose(bool disposeManagedResources)
{
// process only if mananged and unmanaged resources have
// not been disposed of.
if (!this.disposed)
{
Console.WriteLine("ClassBeingTested: Resources not disposed");
if (disposeManagedResources)
{
Console.WriteLine("ClassBeingTested: Disposing managed resources");
// dispose managed resources
if (img != null)
{
img.Dispose();
img = null;
}
}
// dispose unmanaged resources
Console.WriteLine("ClassBeingTested: Disposing unmanaged resouces");
disposed = true;
}
else
{
Console.WriteLine("ClassBeingTested: Resources already disposed");
}
}
// loading an image
public void LoadImage(string file)
{
Console.WriteLine("ClassBeingTested: LoadImage");
img = Image.FromFile(file);
}


}

So can any one tel me tat the reason behind of implimenting dispose method from IDisposable interface. I think everybody understands my question. i searched in google a lot. but i did not get a proper answer.So please clarify my doubt.

Regards
Lijo