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You have to remove alpha channel when uploading a photo to App Store Connect. There is a set of photos of different sizes in
AppIcon image set of
Asset in iOS project, you need to check every one. The images are also in the
APPIcon.appiconset folder ,each of which needs to be replaced.
About unticking the Alpha channel, you could try to open the icon image in Preview in your Mac, select Export under File menu, uncheck Alpha at the bottom, and click Export.
Some other drawing software can also untick the Alpha channel, you could have a try.
About APP icon format, you could refer to
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