You don't need to delete your account just to get rid of certain data from Yandex services. You can delete old photos, get rid of spam, or change your security question using whatever service stores that data. You can find solutions to common problems in the help section of the service in question. All services are listed on the Help main page.
Go to Delete Account on the page containing your personal info to completely delete your account. You will need to confirm that you want to delete it by answering the security question.
If the answer to the security question that you entered is incorrect, check that you are using the correct keyboard language. On the Edit Security Question page you can try different spellings of your answer or different date formats (in the case that your answer contains a date) to try to recall what you originally entered.
If this doesn't help, you will have the option to fill out a special form when you send your error notification so that Support can help you.