Autoptimize & shortpixel image compression

Hi,

after activating “compress images”: (screenshot: https://gyazo.com/7478f8df4b01520488b0c5b77fe5ef46) i got the recommandtion on gtmetrix:

The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources: (screenshot: https://gyazo.com/bbe4cb6077f31be103fdbb3c90c4ca20)

and my score moved from 99 to 96.

How can i eliminate it?

Thank you in advance!

Christian

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