Although it's not listed on that page, I'm fairly sure that these image uploads will also work as described in your second link.
If you explore the docs starting from your first link (i.e. GameCenterAchievementImageResponse -> GameCenterAchievementImage -> GameCenterAchievementImage.Attributes -> uploadOperations), you'll get to "Upload instructions for assets such as app previews and app screenshots". Use this as described in your second link, under "Upload the Asset", to perform the actual upload.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect API
Tags: