My collection images are not watermarked. I see Pinterest as a careless community of copycats. I sell art prints and don't want it pinned without watermarks. My product photos are watermarked, just not the collection images.
The solution depends on theme, any customizations, or apps in use.
Also whether this is an image added to the collection itself or embedded in a description
If embedded in a description use the view source to manually insert the code for the image tag.
For themes generally in your themes admin myshopify.com/admin/themes
Note: this is for themes on the online channel and will in no way automatically add this opt-out attribute to other channels(twitter,facebook) that may consume images on your collection data, always verify.