Thank you for your feedback.
Currently we do not support place an image over a dwf/dwfx file directly.
But you can open images and save them as new DWF files firstly, and then import the DWF file as a Symbol Catalog.
About the detailed steps, please refer from Help document.
1. Open image files: Contents tab -> Using Design Review -> Open Files -> About Opening Other File Types ->
2. Import a DWF file: Contents tab -> Using Design Review -> Markup DWF Files -> Markup 2D DWF Files -> Use Symbols -> Import a DWF file as a Symbol Catalog.