Midjourney subscribers own all the images they’ve created, even if their subscription has expired, and they’re free to use those images however they’d like. There are two small exceptions:
If you upscale an image of another user, that upscale is owned by the original creator, not by you. It will appear in their gallery on the website instead of yours, and you’ll need their permission to use it.
If you are a business grossing more than $1,000,000 USD a year, you need a Pro or Mega Plan to use your images commercially for your company.
Full details can be found in the Midjourney Terms of Service, Section 4: Copyright and Trademark.