Thank you for using this addon. I have investigated the matter. As far I can verify avif images are converted including those of the site you mentioned. However this site uses a protection to avoid hot linking. To avoid that other websites show images that they are hosting. This causes an issue since it also interferes with the download mechanism of the addon. Fixing this is hard since these kind of measures are site specific. However downloading them reopening and than converting works though this approach defeats the purpose of a single step solution.