How wikipedia can use advertising but remain non profit and neutral

Wikipedia has long being against advertisement on it's sites. It depends on users to donate. Users who are mostly below 16 years doing their homework. Wikipedia hardly manages to get any donation from users but some corporate samaritans do help. Instead, they could just start selling advertisement and use the required money for survival and donate the rest to charity. And as for neutrality they can avoid contextual ads.
They could just put one small image ad on every page of one company. Like only At&t ads on every page regardless of content. It can be pay per day basis. Just imagine how much big companies will pay to have their ads on every page of wikipedia even or a day considering wikipedia's traffic.

Here are few of the solutions which was proposed
* Ads on articles pages is the most widely rejected solution, though there are a few supporters
* An opt-in system. Readers will have the opportunity to say "yes, I want advertisement". This would likely require the reader to be identified by an account, and to login. Most readers are unlikely to have an account, so it is not clear this solution would bring much money.
* An opt-out system. Readers will have the ads by default, but can request not to have the ads displayed. Most editors also feel this solution intrusive and it is unlikely to be retained
* Advertisment on search pages. This may be the least controversial solution, as it can not be said to damage our concept of neutrality, and might on the contrary bring a benefit to the reader. It has not been approved though.