Changing the hero icon.

To be honest, I've never worked with Photoshop or alpha channels. At the time, the simplest method seemed to be taking an empty icon that I found somewhere in data.pak; it's the right size and has the right black transparency. I copy this file, then create a second layer for the image and insert what shouldn't be transparent (visible pixels) into the second layer. And nothing more, so that it doesn't overlap incorrectly and result in incorrect transparency.
This seemed like the least troublesome method compared to searching for "incorrect" pixels.
Accordingly, the black frame can also be drawn on the lower, transparent layer.

Of course, this method works not only with an empty icon but also with any non-empty icon where all the pixels that you want to be transparent are already transparent. But it's more universal to find an empty one.

Well, if you get suggestions for other methods, that's great; maybe it will be even more convenient.)
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And one more thing, if needed, you can determine the color of a pixel in any editor after merging the image, using the eyedropper tool on the desired pixel. There you can see if it's black or white (it seems that it should be transparent in any case).
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In general, this description sounds super complicated, but in reality, it's... Simpler. So, if it's still relevant, tomorrow I'll try to make some screenshots in Paint.NET showing how I do it.