As far as I know we can use D-noise tool just after rendering. I wonder if we can use it loading an already rendered image instead.
As we can clearly see on https://remingtongraphics.net/tools/d-noise/ after entering "d-noise" on pretty much any search engine and clicking the first result it says:
D-NOISE isn’t exclusive to renders like the Cycles denoiser is. D-NOISE can run on any image loaded into the UV/Image Editor including texture bakes and even photos!
And here is a screenshot of that:
You can try using this tool instead.