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The export mode of the image is controlled by the ImageExportMode property. For example you can set this property along with the export settings like this:
HtmlFormatProvider htmlProvider =
HtmlExportSettings settings =
settings.ImageExportMode = Telerik.WinControls.RichTextBox.FormatProviders.Html.ImageExportMode.UriSource;
htmlProvider.ExportSettings = settings;
More information about this is available in the following article: Import/Export Settings
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