Is it illegal to have a website were you pay money to see a woman in her birthday suit but the pictures that are posted on the site are not of her in the nude they are pictures of her in a suit?

No. It is not false advertising if the language is specifically "birthday suit". The woman is in a suit, just not the type of suit that you thought it was. This is a function of language and innuendo and is not illegal. Therefore, you paid to see an image where the language is clear...birthday suit...not nudity. You lost, it's legal.