Thank you for your question.
You can add an image to layout of DWF by following steps.
1. Create a new W2D file;
2. Parse all 2D objects out from original DWF file and write them into the new W2D file and add the image at the right place you want. For detail about adding an image into new W2D file, please refer to the sample project "SimpleEPlotWriter";
3. Remove the 2d graphics resource from original DWF file and add the new W2D file as 2d graphics resource.
Please let me know if you have any further questions.