Lustful, idolatrous, or sexual thoughts. But only if you are actively thinking them will they occur. Jesus said that if you look on a person (a picture is just as good) with lust, you've committed adultery in your heart, so he recommends to repent.
But for the majority, it is not a sin to look at the pictures.
I don't think it is in of itself, it all depends on what your response to them. For example I could walk by the beach and see a woman in the clothing described and just keep moving on. I don't think that would be wrong. But if I saw a woman in such clothing and reacted in a sinful way, that is when it would be wrong.
I've heard it like this "It isn't your eyes--it's your heart" so it ultimately depends on how your react to what you see that determines whether it is sinful or not.
EDIT: I could also say it comes down to your intention as to whether or not looking at these images would be wrong. If your just looking to buy underwear and you see it on a model, I really don't see a problem with that. But if your are looking at the model for sinful reasons, that would likely be wrong as well.