Cool, I think the key is “layers”. When I figure out that and get a decent rendition, I’ll post it and have you guys look at it. It’s a nice photo we took coming back from the Blues festival at Sun Banks in central Wa.
Yes, PaintDotNet does layers.
You can copy the sepia photo into one layer and the color photo into another. You can set transparency levels on the layers individually for some interesting effects. (When you save the final image as a jpeg photo, the layers are merged into the file that’s generated.)