I'm creating a fillable PDF template, and I was trying to figure out a way to make it easy to add an image to a defined area, almost like a smart object in Photoshop. I'm creating it for someone who only has an unpaid version of Acrobat Reader, so they wouldn't be able to do anything like "Edit Object" to place an image into the PDF. I'm thinking that this isn't really possible but I don't know if there is a solution out there to do this, because I couldn't find one.
If you are working in Adobe Illustrator, Adobe InDesign or Affinity Designer you can set up PDFs with layers... and your end user can choose which layers are on (visible) and off (hidden) and if you place your images in the correctly registered location, but manage visibility through layers.... I'd think this could work.
We do this sort of thing a lot in architecture with PDFs of large multi-layer drawings and diagrams - works well for us as an entire industry.