If I'm a copyright holder who feels like my work is being misappropriated by Pinterest, I'm going to sue Pinterest, not the user. Their Terms of Service won't stop them from getting sued, and the indemnity clause won't magically make money appear in their pockets to pay for their defense. If they decide to start suing their users for recovery, that would be pretty amusing.
"I trusted the person who gave me the image" is not a legal defense for copyright infringement. Their only chance is to stay within the DMCA safe harbor or else they will eventually be shut down.
As my lawyer explained to me long ago, who eventually "wins" a lawsuit is rarely interesting. Cost, time, and agony to get there are much more relevant factors.
The "our users represent that the content is theirs" may not keep Pinterest from losing an eventual lawsuit, but it does complicate things enough that it discourages legal action. That may be sufficient for them to cash out long before the suits are complete.
Or, like YouTube, things like that may allow them to grow big enough that they end up with sufficient negotiating power that they can get away with quite a bit, and possibly reshape what's considered reasonable.
"I trusted the person who gave me the image" is not a legal defense for copyright infringement. Their only chance is to stay within the DMCA safe harbor or else they will eventually be shut down.