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 SlideShare 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 SlideShare using a third-party login such as Facebook or Twitter, you probably want to revoke all access from SlideShare. Click the icon below to find out how.