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I found this great post that solved two problems for me. Turns out that Let's Encrypt is already part of my Synology Diskstation NAS, it explains how to set it up:

https://stefandingemanse.nl/how-to-use-lets-encrypt-ssl-certificate-on-synology-dsm/

So my problem is solved, my images are now working! Thanks again for tip @pharesim .

Thanks! Will definitely give it a try.