Is it permissible to continue working in the internet field, which involves uploading non-prohibited images, if the platform displays advertisements for pornographic websites without the worker's consent, and is what the worker earns from this work considered unlawful gain?

You must advise the one you deal with to repent. If he does not respond and you are unable to compel him to abide by the terms of the contract that prohibit unlawful acts, then it is not permissible for you to continue working with him, as this would be considered cooperation in spreading indecency. Allah (the Most High) says: “And cooperate in righteousness and piety, but do not cooperate in sin and aggression.” And His saying: “Indeed, those who like that immorality should be spread among those who have believed will have a painful punishment.” The limit of necessity does not apply to your situation, as you can search for another permissible job. And know that whoever abandons something for the sake of Allah, Allah will compensate him with something better than it.