If you search for "add image to pdf" on Internet, you will find many useful articles. However none of them meet my requirements.
I want to add an image to a certain place inside an existing PDF file, for instance incide a textbox.
I am not certain of how exactly you require an image added to your PDF, but there a number of approaches you can consider:
1- Load the PDF as a rasterized image and draw the image at your desired location.
2- Add the image as an annotation to the PDF.
3- Convert the PDF to a format that allows easy modification of text and insertion of images.
Loading the PDF as a rasterized image is the most direct approach. However, your text will no longer be searchable and any other PDF objects (Annotations, Hyperlinks) will all become part of one image (no longer objects). But using this approach you can simply draw the image at the exact place you need. If you want to restore text searchability after doing this, you can use an OCR engine to process the text in the resulting image.
The ImageMagick library uses the Ghostscript common engine for dealing with PDF, and it can convert PDF pages to images. There's a .NET wrapper for ImageMagick to use with C#. For OCR, there are free engines like MODI or Tesseract.
Adding the image as an annotation allows you to maintain the original format and text in the PDF, though the image will be treated as a separate object than the text and will not be “in-line”. Annotations also allow you to draw them at the exact location you need without too much difficulty.
LibreOffice Draw and Okular are options you can consider for drawing annotations.
Finally, you could simply convert the PDF to a format that easier for processing and editing, like DOC, add your image then convert it back to PDF.