First of all, you have to be sure which you seem to be. The only solution is to confront her and get to the root of why this behaviour occurred in the first place. Also a naked picture on the Internet does not imply that she is interested in sex, so it does not imply that she has been pretending with you. Instead if she doesn't have a history of such behaviour then maybe you should discuss it and resolve the matter as soon as possible. You need to remember that it may not be a recent picture and could have been taken in different circumstances, since you know your wife and her past you will be the best judge of how to confront this issue. So just keep an open mind and do not accuse her, instead be open and empathetic in your responses. Most of all, its important for you to try and remain calm. If a bigger interpersonal issue emerges, then marriage counselling is suggested.