I have done this before several ways which is why it is very frustrating now.

I read all of Add transparency to an existing PNG but didn't try the "Curves on the Alpha Channel" suggestion from Demis. It's usually enough to (1) Add Alpha Layer and (2) Color to Alpha (2) Export as png or jpg.

I have other images like this one that upload correctly: https://oldcrowhempco.com/shop/green-lotus-hemp-heat-rub/

But then this one I put through the same process, it looks it has trans. backgrounds, but then when uploaded, it's displayed with white background: https://oldcrowhempco.com/shop/green-lotus-premium-hemp-oil-salve-250-mg/

One thing I noticed that I thought was odd when I downloaded the one that's appearing correctly and imported back into GIMP, Mode is Indexed not RGB. However, while I have not been able to capitalize on this knowledge yet, it seems relevant.



Not a complete answer but too long for comments:

1) I can use wget with the URL with parameters to obtain an image which is a color-indexed PNG (60K). But it looks OK (transparent background).

2) Through my Firefox browser, both parameterized and paramater-less URLs are the same 20K image (binary equal) which, despite its name, is a WebP and not a PNG, so there is likely some re-compression going on. This image is also OK in Gimp and in Firefox. However being a WebP it may not be supported adequately with MS browsers. So if you are using Edge this could be part of the problem.

Color-indexed images in Gimp have a "binary" opacity, pixels are either fully opaque or fully transparent. But I don't thing this is what is happening there.

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