That's a very basic view, take a look at TIFF or DICOM specs. It can be useful to have multiple images, resolutions, channels, z or t dimensions, metadata, ... all in a single container as it's all one "image"
captions / alt-text could also very reasonably be part of the image, as well as descriptions of regions and other metadata.
there are LOTS of uses for "image containers" that go beyond just pixels. heck, look at EXIF, which is extremely widespread - it's often stripped to save space on the web, but it's definitely useful and used.
