An 1023x766 image displayed in a 256x192 thumbnail. PageSpeed will resize it and also serve with a srcset for other sizes.
An 1023x766 image displayed at full size. PageSpeed may recompress it, but will not add a srcset.
An 1023x766 image displayed at 682x511 (2/3 scale). PageSpeed will resize it and also serve a 1.5x version in srcset (but not 2x since that would be redundant).
A small image. PageSpeed will inline it and not set a srcset.
A medium size image with no listed width nor height. PageSpeed will losslessly re-encode it, but not resize it nor add a srcset for other sizes.
An image with data-pagespeed-no-transform. PageSpeed will not transform this file.