The Image Map Dialog is not rendered correctly because of a failure of the CSS reset. There is a margin-top attribute which is set with 15px value instead of 0.

Possible resolutions:
 1. Setting another Skin for the RadEditor control;
 2. Setting a custom stylization for the dialogs, which fixes the margin issue. For this approach you can test this example setup along with the provided DialogStyle.css file:

<telerik:RadEditor ID="RadEditor1" runat="server" 
	DialogsCssFile="~/Styles/DialogStyle.css">
</telerik:RadEditor>

More information about the DialogsCssFile property: http://www.telerik.com/help/aspnet-ajax/editor-dialogscssfile.html