Thanks, Bozhidar:
I was able to incorporate two separate images for the handles by manipulating the CSS as follows:
Default.aspx:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="Default" %>
<head runat="server">
<title></title>
<telerik:RadStyleSheetManager id="RadStyleSheetManager1" runat="server" />
<style type="text/css">
div.RadSlider_Black .rslHorizontal .rslDecrease
{
background-image: url('images/minus_icon.png') !important;
width:27px !important;
height:27px !important;
background-position: bottom left;
}
div.RadSlider_Black .rslHorizontal a.rslIncrease
{
background-image: url('images/plus_icon.png');
width:27px !important;
height:27px !important;
background-position: bottom left;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
<Scripts>
<%--Needed for JavaScript IntelliSense in VS2010--%>
<%--For VS2008 replace RadScriptManager with ScriptManager--%>
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.Core.js" />
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQuery.js" />
<asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQueryInclude.js" />
</Scripts>
</telerik:RadScriptManager>
<script type="text/javascript">
//Put your JavaScript code here.
</script>
<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server">
</telerik:RadAjaxManager>
<div>
<telerik:RadSlider runat="server" ID="RadSlider1"
Value="50" Skin="Black" >
</telerik:RadSlider>
</div>
</form>
</body>
</html>
See attached images ("minus_icon.png" and "plus_icon.png") and resultant output ("Custom_RadSlider_Handle_Images.png").
Hope this helps!