The Image Properties dialog is loaded in the same way as the other editor dialogs (e.g. Find and Replace, Table Properties, Link Manager). Are you sure that it is the only one not working in your site? If the other dialogs work, see what URL they are using (is it the same as the one in the error - /xxx/DesktopModules/SGN_News/Telerik.Web.UI.DialogHandler.aspx).
Are you using the RadEditor provider for DNN or the normal RadEditor control? If you are using the editor provider, try copying the Telerik.Web.UI.DialogHandler.aspx file from \DesktopModules\TelerikWebUI\RadEditorProvider\ to the \DesktopModules\SGN_News\ folder.
the Telerik team