private void radRichTextBox_CommandExecuted(object sender, CommandExecutedEventArgs e)
{
    if (e.Command is DeleteCommand && imageToDelete!=null)
    {
        this.radRichTextBox.Document.Selection.Clear();
        this.radRichTextBox.Document.Selection.AddDocumentElementToSelection(imageToDelete);
        this.radRichTextBox.Delete(true);
        this.imageToDelete = null;
    }
}

private ImageInline imageToDelete;
void radRichTextBox_CommandExecuting(object sender, Telerik.Windows.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is DeleteCommand)
    {
        var selectedBoxes = this.radRichTextBox.Document.Selection.GetSelectedBoxes().ToList();
        if (selectedBoxes.Count() == 2 && selectedBoxes[0].AssociatedInline is FieldRangeStart && selectedBoxes[1].AssociatedInline is ImageInline)
        {
            imageToDelete = selectedBoxes[1].AssociatedInline as ImageInline;
        }
    }
}