So, Cosine, during all the years that I photographed bikini models for Snap-On Tool posters, I was engaging in some kind of sin?
I'm not in the position to judge; but I'll make a comment.
It would depend upon a few things (the extenuating circumstances I keep mentioning). Firstly, it would depend upon the intent and purpose for which the poster were being made. The moral culpability (of which I am not to judge) of that would be mostly upon the company for whom the posters were being made.
Secondly, it would depend upon your attitude, such as your engagement of such a job in order to deliberately further the objectification of someone's human body for sexual pleasure versus your engagement of such a job simply because it is a means of work and a means of making a living.
Thirdly, the sinfulness of posters such as those would depend upon the attitudes of those who received them and viewed them, whether they intended to use them as a means of sexual pleasure (both physical or imagined) or not.
And, like usual, all extenuating circumstances also would be determent upon your understanding at the time of your actions and their potential or non-potential sinfulness or even whether you considered your actions sinful or not.
As you see, there are quite a few crucial points to be hashed out before a judgment of your moral culpability could be made, and then you firstly would be the one capable of doing so.
Hope you aren't offended by this post; such is not my intent. My intent is only to try to answer your question as well as I can.
Another point, a bit off-topic: The standards of modesty differ between cultures. For example, there are cultures in Africa where women normally go topless. In that case their actions are not even objectively sinful, because their culture is different than our (the breasts are not seen as being as much of a sexual part of the body as in our culture), and the intent and purpose of such exposure is not to be titillating and provocative, as the exposure of such organs are normally seen in our current culture. Such a manner of dress is ingrained in their traditions.