If the images are also custom (not these that are part of Telerik.Web.UI assembly), then it sounds like a permission issue to me. If the defaults are not displayed (and they should be), then there must be something different in the rendering or the stylesheets evaluating. Could you, please, send us a sample project containing your custom skin?
Perhaps on the server somehow the image is made inaccessible for client requests. You can inspect this in the browser with Firebug (Firefox), Developer Tools (called the same in Chrome and IE browsers). In the network section you can search for the request made for the images and check the status.
the Telerik team
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