Replace image in word doc using OpenXML

Although the documentation for OpenXML isn’t great, there is an excellent tool that you can use to see how existing Word documents are built. If you install the OpenXml SDK it comes with the DocumentReflector.exe tool under the Open XML Format SDK\V2.0\tools directory.

Images in Word documents consist of the image data and an ID that is assigned to it that is referenced in the body of the document. It seems like your problem can be broken down into two parts: finding the ID of the image in the document, and then re-writing the image data for it.

To find the ID of the image, you’ll need to parse the MainDocumentPart. Images are stored in Runs as a Drawing element

<w:p>
  <w:r>
    <w:drawing>
      <wp:inline>
        <wp:extent cx="3200400" cy="704850" /> <!-- describes the size of the image -->
        <wp:docPr id="2" name="Picture 1" descr="filename.JPG" />
        <a:graphic>
          <a:graphicData uri="http://schemas.openxmlformats.org/drawingml/2006/picture">
            <pic:pic>
              <pic:nvPicPr>
                <pic:cNvPr id="0" name="filename.JPG" />
                <pic:cNvPicPr />
              </pic:nvPicPr>
              <pic:blipFill>
                <a:blip r:embed="rId5" /> <!-- this is the ID you need to find -->
                <a:stretch>
                  <a:fillRect />
                </a:stretch>
              </pic:blipFill>
              <pic:spPr>
                <a:xfrm>
                  <a:ext cx="3200400" cy="704850" />
                </a:xfrm>
                <a:prstGeom prst="rect" />
              </pic:spPr>
            </pic:pic>
          </a:graphicData>
        </a:graphic>
      </wp:inline>
    </w:drawing>
  </w:r>
</w:p>

In the above example, you need to find the ID of the image stored in the blip element. How you go about finding that is dependent on your problem, but if you know the filename of the original image you can look at the docPr element:

using (WordprocessingDocument document = WordprocessingDocument.Open("docfilename.docx", true)) {

  // go through the document and pull out the inline image elements
  IEnumerable<Inline> imageElements = from run in Document.MainDocumentPart.Document.Descendants<Run>()
      where run.Descendants<Inline>().First() != null
      select run.Descendants<Inline>().First();

  // select the image that has the correct filename (chooses the first if there are many)
  Inline selectedImage = (from image in imageElements
      where (image.DocProperties != null &&
          image.DocProperties.Equals("image filename"))
      select image).First();

  // get the ID from the inline element
  string imageId = "default value";
  Blip blipElement = selectedImage.Descendants<Blip>().First();
  if (blipElement != null) {
      imageId = blipElement.Embed.Value;
  }
}

Then when you have the image ID, you can use that to rewrite the image data. I think this is how you would do it:

ImagePart imagePart = (ImagePart)document.MainDocumentPart.GetPartById(imageId);
byte[] imageBytes = File.ReadAllBytes("new_image.jpg");
BinaryWriter writer = new BinaryWriter(imagePart.GetStream());
writer.Write(imageBytes);
writer.Close();