Image Property

hi,i'm working in a CustonControl, and i want add it an image property.
i create a property for the image:

public System.Drawing.Image Imagen
{
get { return myImagen; }
set {
myImagen = value;
Invalidate();
}
}

this work's fine when i set a value to propertie, but in the IDE of
VS.NET(NET 2.0), when i go to the property tab page of control i can't
delete the value of the property.
if in code, i set a value to Imagen, for example Imagen=null;, the value
of the property delete correctly.

why i can't delete value from property page in the IDE?

thanks