Five images per slide are generated, the used image settings are below, on the advanced tab : Output format JPEG
The first panoramic picture can not be expanded, but the 2nd panorama picture can be expanded (maxImageWidth > imageWidth ).
In the middle of the slide page you see which image has been used on the current device. This is a debug statement which is not displayed in normal albums and also not if srcsets are not used for the current device.
The album is made with in FancyBox check-mark 'Make images look sharper on HDPI displays' set, which gives the best result on my 4k HP Laptop. No HDPI selected in jAlbum except for thumbs.
Another FancyBox test album: 1920-3imgs/slide
jAlbum image settings: