Am I sinful for working as a backend developer for a website that contains pictures of immodestly dressed women, knowing that my work does not involve placing these pictures and the product displayed is permissible? Does this fall under aiding in sin, and do I bear the sins of everyone who sees these pictures?

If the website's activity and the goods displayed on it are permissible, then working on developing its backend interface is not forbidden. This is not considered cooperation in sin, even if the frontend developer places an image of a disobediently adorned woman. He should advise the one who placed it and clarify the religious ruling to him.