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I just read this page:
It says that image styles will append a token to the end of image URLs.
As an example, links that previously pointed to a URL like http: //example.com/sites/default/files/styles/thumbnail/public/field/image/example.png will now point to a URL like http: //example.com/sites/default/files/styles/thumbnail/public/field/image/example.png?itok=zD_VaCaD.
If a site changes the URLs of images, it can be bad for SEO. I've had Drupal sites with 1,000+ visits per day just from image searches, so image SEO is an issue that should be considered.
Maybe there is another way to fix the security problem other than changing URLs? URLs should never change...