It's an unrealistic test as pages aren't really made up of tiles of images
Generally images are the lowest priority download, so ensuring higher priority items get downloaded first is important and not all H2 implementations do it well
Generally images are the lowest priority download, so ensuring higher priority items get downloaded first is important and not all H2 implementations do it well
Images are the lowest priority resource on the page and apart from the visual appearance aspects there are no dependencies on which order they're fetched in.
Most other resources on a page have greater side effects and and dependencies e.g. sync JS blocking the HTML parser, sync and deferred JS need to be executed in order etc.
You can saturate a last mile connection with image downloads in a way you man not be able to with other resources due to effect of the browser processing those resources.