metallissimus wrote:The thing is: I do have images in my index folder. They are in webp format though, so I guess they don't get recognized by diagnostics?
That's correct, you are all good. It's just that diagnostics was not updated to take into account webp format. This warning comes up for the home page only, because it's important that you at least have one preview image available there.
After updating to the latest version, I get the following warning in diagnostics for my wedding photography site:
FYI, yhis warning would probably have shown before updating also.