All the photos I use are creative commons licensed, but that requires me to credit the photographer. If I allow it to be pinned on Pinterest, it's likely the credit won't be included when it's repinned. That means I'm wilfully setting up a situation where the license is likely to be breached, and I'm not comfortable with that.
Besides, as I've just discovered, links on Pinterest are now "No Follow" - so if the purpose of pinning is to get backlinks, it's no longer effective.