theme options- feutured slider Photofocus Pro-JPEG to WEBP

Hi, I use the PhotoPhocus Pro and I try to put WEBP photos in teh Featured slider via Theme Options. I see the photos in the left bar of the cusumizer but they dissapear after publishing. So I can put WEBP format pics everywhere I want in the template only not in the Featured slider. I want that to optimize more my website. What is the solution? Thanks for a reaction, kind regards Peter