title: itext7 PdfButtonFormField setImage method does not work on a signed pdf
body: <p>I am using itext7 java library as shown below to add PdfButtonFormField to an existing pdf :</p>
  
  <pre><code>String src = "sample.pdf";
  String dest = "acro_sample_empty_fields.pdf";
  
  
  PdfDocument pdf = new PdfDocument(new PdfReader(src), new     PdfWriter(dest));
  PdfAcroForm form = PdfAcroForm.getAcroForm(pdf, true);
  PdfButtonFormField button = PdfFormField.createPushButton(pdf, new Rectangle(Integer.parseInt(control.xCord), Integer.parseInt(control.yCord),Integer.parseInt(control.width), Integer.parseInt(control.height)), control.name, control.value);
  
  form.addField(button, page);
  
  String resource = "sample.png";
  button.setImage(resource);
  </code></pre>
  
  <p>After this i use the following code to fill the form like below :</p>
  
  <pre><code>String src = "1540982441_313554925_acro_sample_empty_fields_signedFinal.pdf";
  String dest = "acro_sample_filled_fields.pdf";
  
  PdfReader reader = new PdfReader(src);
  File output = new File(dest);
  
  OutputStream outputStream = new FileOutputStream(output);
  PdfDocument document = new PdfDocument(reader,
                      new PdfWriter(outputStream),
                      new StampingProperties().useAppendMode());
  PdfAcroForm form = PdfAcroForm.getAcroForm(document, true);
  Map&lt;String, PdfFormField&gt; fields = form.getFormFields();
  
  String resource = "sample_test.png";
  ((PdfButtonFormField)fields.get(control.name)).setImage(resource);
  </code></pre>
  
  <p>Everything works fine for a normal pdf. But if i digitally sign the created pdf and then try to fill it. then the image is not set properly. 
  For a normal pdf the image on the push button is changed as expected. But on the digitally signed pdf the image is not set.</p>
  
  <p>I have tried looking for this on google but no luck yet. Any help will be appreciated. Thanks in advance.</p>