I can understand that your husbands behaviour may upset you. I expect you may be feeling let down, that he prefers those images to you and that somehow you dont measure up to what turns him on in a woman. However, all that said, Providing he isnt going any further than viewing the images, there shouldnt be a problem.
That is only my perspective, as if it happened to me (and it has).
To some men im sure looking at those images, is a bit of escapism and a bit alternative. If you are feeling threatened by him doing this, maybe you and he should sit down and discuss what it is he gets from looking at the images. You need to be honest and tell him how you feel. I suspect you feel insecure and i also suspect you and he dont communicate your feelings to each other in an open and honest way. Im sure he doesnt want to hurt you, which is why he tries to hide them from you. Good luck. Hope this is a little bit of help