Thanks for the prompt reply!
By just calling printHDDefaultSizedImage() I'm making progress -- I see the HTML below served by ZP. BUT, the image looks exactly the same. The source of the image is definitely high enough resolution. Should I set any other settings in ZP, or did I mess up somewhere else ?

`

`

Ah, sorry you have to set the 2nd parameter to true. So printHDDefaultSizedImage(null, true). For some reason I don't remember the HD image is false by default.The parameter makes no real sense actually… Will review that for the next version. That by the way will not use `for the simple SD/HD image ones but the lighter ´.

That worked, thanks! Now my album & image pages are showing nice retina-friendly images.
... now how do I hack slideshow2 to do this too?

You will actually have to code your own plugin version. Look at the Cycle class and its method getSlide. There you have to replace the image calls with the picturefill ones (best with a check so it does not break if the plugin is off).

Or you use the workaround of providing bigger image sizes with more compression while scaling them down.

That often is enough for example if you have a responsive theme where images are scaled smaller anyway on breakpoints.