Thanks for the prompt reply, here are the screenshots requested:
https://imgur.com/a/KeChuei
WebP express seem to work with the following config:
https://live.staticflickr.com/65535/51432146411_b15575b503_o.png
nevertheless that is not a LayerSlider issue/bug since replacing the img tags is not a feature of their plugin but rather this one 🙂
