Is money earned from designing signs for stereo or video stores considered unlawful (haram), and am I sinful for it?

The fundamental principle concerning contracts is permissibility. However, if a contract is accompanied by a form of assistance in wrongdoing, it becomes prohibited, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." Therefore, it is not permissible to sell something when it is known that the buyer will use it for a sin, such as selling juice to someone who will turn it into intoxicants, or selling weapons during a time of strife, or selling tools for committing forbidden acts. Accordingly, it is not permissible to sell or create paintings for someone who will use them as a means to commit forbidden acts, like selling forbidden tapes.