Pinterest allows someone to collect images of things they like and organize them on virtual bulletin boards. Great idea. The problem is that unless the person doing the pinning owns the image, the real owner might object to the use of their property. Think of it this way, my car has the technical capacity to drive 60mph in a 30mph zone. That doesn't make it a good idea. Pinterest gives us the technical capacity to take images that are owned by someone else and put those images somewhere for our own use. That technical capability doesn't make it a good idea.
The Pinterest terms and conditions aren't really helpful in determining what you can and can't do, not because they don't want to be helpful, but because they can't speak for the people that you are taking the images from. It's like my asking your neighbor if I can park in your garage. The neighbor doesn't have the authority to speak on your behalf about your property.
Copyright law states that the owner of the copyright has the exclusive right to copy, distribute, display, perform and create derivative works from their work. In order for a use to be "fair use" under the US Copyright Act certain criteria must be met, most of which aren't likely to apply to the pinning of an image. Some copyright owners may, on their website, tell you that you can or can't pin something on Pinterest, but most won't. Putting an image on the internet does not, by itself, imply that someone else may put that image on Pinterest.
Trademarks can't be used in a way that creates a likelihood of confusion as to the source of the goods. This means that you can't pin images containing trademarks in a way that makes it look like you or your board has an association with the trademark owner.
Right of Publicity, Celebrity Images, and Children.
There are a number of laws that govern the use of someone's image. The general theory is that no one gets to use your face without your permission. We are particularly sensitive about the use of celebrity images and images of children. Celebrities because they earn their living from their images, and children because of their vulnerability.
Pinterest's terms and conditions make the user responsible for determining whether they can pin an image, and if you pin something that gets Pinterest sued, you agree to pay all of Pinterest's expenses in defending the suit.
What to do? First, know that most copyright owners aren't worried about your pins, you may even be helping the owner sell more products, or get more views. But, some pins are riskier than others. As a group, people who make their living from selling their images, (e.g. photographers, painters), are more likely to object to being pinned. There is both an artistic integrity issue and a loss of control issue underlying their objections. Images containing children and celebrities can be higher risk. If you are a business, and you are using your boards to imply celebrity endorsement through use or imply something about your competitors, the risk is higher.
And finally, if you do receive a cease and desist letter take it seriously and consult with counsel experienced in the area of the law described in the letter. There's a lot of information on the internet about the law and not all of it is accurate. The challenge isn't in reading a statute, but in applying that statute to your facts. Different facts can have different outcomes, even under the same statute. And the application of the law to particular fact patterns isn't always intuitive.