Six IPs, same UA, or six UAs, same IP? (Thread title != post content.)
Are you sure it's pinterest? What are they doing, splat in the middle of AWS territory where everyone gets 403'd? Normal pinterest pattern is to pretend they're getting your whole page, thereby bypassing anti-hotlinking routines, and then display only the image. You have to physically look at their page to figure it out. (One of the rare occasions I've gotten something useful out of google's link list.)
Error in post content, I meant to write IP not UA.
I see the trap I fell into. I have the alleged pinterest UA banned and concluded that was the reason for the 403's. Checking my .htaccess, I see that I had indeed banned all the AWS IP's listed. Hmm...