@s2b As someone who has also struggled with this and also created a TYPO3 extension for it i completely agree with: "a complicated subject with a lot of historical baggage". Things have gotten a bit better in the last years with stuff like intrinsic ratio, but still not an easy topic.
Btw: i came to the conclusion to call them adaptive images.
Responsive: resizes with viewport (max-width: 100%)
Adaptive: deliver the right image for each device.