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@Matt Wilcox – Regarding the cookie issue, the AI cookie will not change, but in real life scenarios, the user might have other cookies as well (session ID, etc). “Vary: Cookie” means that the resource is invalid every time any cookie value is changed for this resource. If you have user specific cookies (again, session ID is the first one that come to mind), the resource is effectively non cacheable.
All in all, it is complicated to get the images cacheable under the “URL must remain unchanged” assumption.