Hi Liya,
You can replace the sign out button with image control in CMS -> CMSAdminControls -> UI -> UserMenu.ascx.
<cms:LocalizedLinkButton runat="server" ID="lnkSignOut" OnClick="lnkSignOut_OnClick" ResourceString="signoutbutton.signout" EnableViewState="false" />
to
<asp:ImageButton runat="server" ID="lnkSignOut" ImageUrl="<image-url>" EnableViewState="false" OnClick="lnkSignOut_OnClick"/>