Right now, I do believe that potentially showing bad ads could be a risk.
In the end, though, I do not believe that web-sites will be held immediately responsible for random ads, but will be found to fall under the DMCA safe harbor, if they are ads randomly pushed. (See [chillingeffects.org
Codified as section 512 of the Digital Millennium Copyright Act (DMCA), this new law exempts on-line service providers that meet the criteria set forth in the safe harbor provisions from claims of copyright infringement made against them that result from the conduct of their customers. These safe harbor provisions are designed to shelter service providers from the infringing activities of their customers. If a service provider qualifies for the safe harbor exemption, only the individual infringing customer are liable for monetary damages; the service provider's network through which they engaged in the alleged activities is not liable.
Similar to the safe harbor for web-sites, hosting providers, and ISPs, for ads that are randomly pushed to a web-site, I think sites can claim to be deemed a "service provider", and get the initial free option of taking action to simply seize and desist. As in filtering out those ads from appearing. The infringement claim then goes against the entity that created the ads, where it really belongs.
Singular ads, where you have a direct agreement with a third-party to show very specific, known-up-front ad content, I think more due diligence is needed to make sure there is no violating content.
It is a fine line.. Sort of similar to the difference between sites that allow random user content to be uploaded, which might
be in violation, and sites such as Pinterest, which more than just allowing random content directly provide the tools for its users to "steal" content and images from web-sites. Which surely mean that they know there is a very high likelihood of being a violation.
One of the reasons I do not believe that Pinterest in its current form will survive for long. Suits from companies such as Petty Images and others will stop their practices.