We all have fantasies. Problem is when you want to put those fantasies into action. What can start of as fun, can end in something very unpleasant. Have you thought about being blackmailed? What if those pictures are circulated among your family or in the office?
Dont presume that your secret will always remain a secret. Even if you eraae the pictures from your PC or laptop, chances are they are still in memory and can be retrieved by a professional, whenn you repair your camera or electronic device.
When you are in bed alone with her, you can fantasize to your heart's content. Do that if it really makes you feel horny. But keep it at that. There are even greater problems with sharing your partner for a threesome or groupsex, but we will leave that topic for another day.