Add cover image for 15.9 release post

Why is this change being made?

This MR was created to add the cover image for just 15.9 and unblock the main Release Post MR: !119153 (merged)

• We noticed that this MR has not merged, which adds all of the images for the marketing release posts:
  • Product#5162
  • Comment: Product#5162 (comment 1265598855)
  • populate /images/release_post_covers/

Author Checklist

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