Currently when cookiless session is enabled and RadButton with custom image is used the Image is not displayed, until the button is hovered.

ASPX:

        <telerik:RadButton ID="ButtunTest" runat="server" Icon-PrimaryIconUrl="~/Examples7/eClose.png" Text="Testbutton"
            Width="200px" UseSubmitBehavior="False" />

Web.Config:

  <system.web>
        <sessionState cookieless="true"></sessionState>
  </system.web>

The workaround is to override the background image of the button:

    <style>
        .RadButton.RadButton_Default.rbSkinnedButton .rbPrimaryIcon {
            background-image: url("/RadControlsWebSite2/Examples7/eClose.png");
        }
    </style>