You have to manually delete all Stack Exchange accounts (like StackOverflow). There's no way to delete all your Stack Exchange accounts in one go.
Publicly visible text/images often aren't properly deleted even when you succeed in deleting an account. Try editing or deleting them manually before deleting the account itself. If you're unsure what happens to your tracks you can always contact Stack Exchange and ask personally. By the way, deletion requests don't necessarily mean your data will actually be deleted (e.g. due to legal obligations).
If you signed up for Stack Exchange using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Stack Exchange. Click the icon below to find out how.