Thanks to the best News Analyst in the SEO community - the one and only u/rustybrick

John Mueller at Google recently confirmed that Google Image Search and Google Discover don't yet support the JPEG XL image format. He said this on Bluesky recently, "FWIW if you're hoping to get images shown in Google Images & as previews in Discover, etc - JPEG-XL is currently not supported."

He then linked to the official help documentation that lists out which image formats Google Search supports, it says:

Google Search supports images referenced in the src attribute of img in the following file formats: BMP, GIF, JPEG, PNG, WebP, SVG, and AVIF. It's also a good idea to have the extension of your filename match with the file type.