how do i change the background image of a button from another form

Hai i have problem i want to change the background image of button that is in another form

i did this but isnt working

// this is the form that im using form1

   menu_ratos f4 = new menu_ratos();
            // imagemlogin is the button that is in form 4
            f4.imagemlogin.BackgroundImage = ((System.Drawing.Image)(Properties.Resources.master));
            f4.imagemlogin.Size = new System.Drawing.Size(14,13 );
            f4.Hide();

// this is the form 4

public Button imagemlogin { get { return Utilizador; } } // i did this to make the button that has the name "Utilizador" public to call it from the first form

SO plis i would appreciate any help thanks!!! really need it