Pinterest is a site where you can make virtual bulletin boards of things you like, recipes,crafts etc. Many sites have a direct link where you can pin things to your boards. I just noticed that this site has a Pin It button on posts with pictures.
I personally think it's really a nice feature. If someone didn't want their photos public then they shouldn't be posted at all on the internet. So for me, I LUV IT :) But I love Pinterest also and yes, I do pin others photos that I enjoy and I know many have pinned my photos that I don't mind having on the internet.
The pin option appears if there are images posted using the [img] or [attach] tags. I am working on a solution to make the pin button appear if a post has attachments that are not posted inline (using the mentioned tags).
I disagree with the likelihood increasing of image being stolen if they are pinned. Everyone needs to be fully aware that any content posted anywhere to the public realm are subject to potential theft or abuse. If you don't want an image to be potentially reproduced in any capacity, don't post it online.
The idea is to leverage the Pinterest social platform as means to expand on social sharing... which is what I assume every poster here is looking for when posting images for the social community to see.