Now that I can make maps in Blender and compile then I thought it was time I started getting back into modeling. I was a bit out of practice and I'd forgotten how to do some of it, so took a while to sort out the configuration, but it's all sorted now.
I took a painting by Peter Paul Rubens (warning, mild nudity) and carefully made a mesh in Blender that was the correct proportions and built a simple frame around it. The picture and the frame are two separate meshes, but they line up perfectly. Nothing fancy, but it works as a proof of concept.