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Couldn't Content Security Policy (CSP) [1] be used to mitigate this attack?

It actually can't. Instagram does use this protect java-script injection from extensions, but clearly injecting CSS is allowed.

But could you use CSP to block the image loading that happens in the CSS with 'img-src' definitions?

Someone else in this thread suggested that as well.

The requested resource, however, could be blocked by a CSP.