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

CSS:

    <style>
        .RadButton.RadButton_Default.rbLinkButton .rbPrimaryIcon {
            background-image: url("images/Groups (16x16).png");
        }
    </style>

ASPX:

       <telerik:RadButton ID="RadButton1" runat="server" ButtonType="LinkButton" Text="Click" Skin="Default">
            <Icon PrimaryIconUrl="images/myImage (16x16).png" PrimaryIconTop="2" PrimaryIconLeft="2" />
        </telerik:RadButton>