DOC: Stamp images directly on a PDF (#2357)

See #1945

Co-authored-by: dmjohnsson23 <dmjohn235@gmail.com>

Author: MartinThoma

docs/user/add-watermark.md

@@ -58,3 +58,56 @@ Example of stamp:
 
 Example of watermark:
 ![watermark.png](watermark.png)
+
+
+## Stamping images directly
+
+The above code only works for stamps that are already in PDF format.
+However, you can easilly convert an image to PDF image using
+[Pillow](https://pypi.org/project/Pillow/).
+
+
+```python
+from io import BytesIO
+from pathlib import Path
+from typing import List, Union
+
+from PIL import Image
+from pypdf import PageRange, PdfReader, PdfWriter, Transformation
+
+
+def image_to_pdf(stamp_img: Union[Path, str]) -> PdfReader:
+    img = Image.open(stamp_img)
+    img_as_pdf = BytesIO()
+    img.save(img_as_pdf, "pdf")
+    return PdfReader(img_as_pdf)
+
+
+def stamp_img(
+    content_pdf: Union[Path, str],
+    stamp_img: Union[Path, str],
+    pdf_result: Union[Path, str],
+    page_indices: Union[PageRange, List[int], None] = None,
+):
+    # Convert the image to a PDF
+    stamp_pdf = image_to_pdf(stamp_img)
+
+    # Then use the same stamp code from above
+    stamp_page = stamp_pdf.pages[0]
+
+    writer = PdfWriter()
+
+    reader = PdfReader(content_pdf)
+    writer.append(reader, pages=page_indices)
+    for content_page in writer.pages:
+        content_page.merge_transformed_page(
+            stamp_page,
+            Transformation(),
+        )
+
+    with open(pdf_result, "wb") as fp:
+        writer.write(fp)
+
+
+stamp_img("example.pdf", "example.png", "out.pdf")
+```