RemoveWatermarkImages.cs

private void RemoveWatermarkImages(TXTextControl.TextControl textControl)
{
    // List of images to be deleted
    List<TXTextControl.Image> lImagesToRemove = 
        new List<TXTextControl.Image>();

    // loop through all images and check for the Name
    foreach (TXTextControl.Image img in textControl1.Images)
    {
        // add watermark images to the List
        if (img.Name == "Watermark")
            lImagesToRemove.Add(img);
    }

    // remove all tagged watermark images
    foreach(TXTextControl.Image img in lImagesToRemove)
    {
        textControl1.Images.Remove(img);
    }
}