If you contribute and upload photographs to popular free stock photography sites, you should opt-out of this AI training setting. Otherwise, these companies can use your creations to train their own models or sell it to another company which does so. I will be covering several sites in this post, please comment if there are other sites worth adding!
Pixabay
Appears in your account settings (after Notifications)
https://pixabay.com/accounts/settings/?tab=account
Pexels
Within your account settings, scroll down to the bottom
Unsplash
This site doesn't currently have a setting to opt-out
(Bonus) Canva
https://www.canva.com/settings/privacy-preferences
I will make a sepertae post for other websites (besides stock photography) that have similar settings