<style>
        .RadComboBox .rcbInput {
            padding-left: 30px;
        }
    </style>
    <telerik:RadComboBox ID="uxIconCombo" runat="server"
        OnClientLoad="OnClientLoad"
        OnClientSelectedIndexChanged="OnClientSelectedIndexChanged">
        <Items>
            <telerik:RadComboBoxItem runat="server" ImageUrl="~/Images/Icon1.png" Text="Item 1" />
            <telerik:RadComboBoxItem runat="server" ImageUrl="~/Images/Icon2.png" Text="Item 2" />
        </Items>
    </telerik:RadComboBox>
    <script>
        function OnClientLoad(sender, args) {
            var input = sender.get_inputDomElement();
            input.style.backgroundImage = "url(" + sender.get_selectedItem().get_imageUrl() + ")";
            input.style.backgroundRepeat = "no-repeat";
        }

        function OnClientSelectedIndexChanged(sender, args) {
            var input = sender.get_inputDomElement();
            input.style.backgroundImage = "url(" + args.get_item().get_imageUrl() + ")";
            input.style.backgroundRepeat = "no-repeat";
            input.blur();
        }
    </script>