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.