Three 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 ).
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 Srcset test album: SrcSets_FB_1920x1920_webP
jAlbum image settings: