I did exactly that, replace oldwinapi with winim and change some definitions around to make it work. I got it to compile after modifying the png package (replacing TFile with File), but I don't have a working png.dll to verify it works.
I also don't know enough about the PNG format/processing to gauge how hard it would be to port it to [nimPNG](https://github.com/jangko/nimPNG) but that would presumably remove the requirement for the dll.
