[jonsneyers]

Containers are just containers — you still need a decoder for their payload codec. This is the same for video and images. For video, containers are more important because you typically have several different codecs being used together (in particular video and audio) and the different bitstreams need to be interleaved.

The ISOBMFF format is used as a container for MP4, JPEG 2000, JPEG XL, HEIF, AVIF, etc.

And yes, there are ffmpeg-like "uber-libraries" for images: ImageMagick, GraphicsMagic, libvips, imlib2 and gdk-pixbuf are examples of those. They support basically all image formats, and applications based on one of these will 'automatically' get JPEG XL support.

Apple also has such an "uber-library" called CoreMedia, which means any application that uses this library will also get JPEG XL support automatically.