Whether images/audio/video material etc. should be considered character, I am doubtful. I currently prefer to summarize them together with formatted text documents and database access (information systems/Identification or query interfaces) or any other URLs as "external resources".
I'd agree here. From the examples I've seen of DELTA files, using characters (or at least their syntax) to describe images is just a convenience - they're not naturally part of the model.
images/audio/video are supporting resources and certainly wouldn't be embedded within the format (although this is possible). I could envisage an audio clip being useful in identifying an item, but a textual description to accompany the resource ought to be a requirement.
This means that anyone using the format 'offline' doesn't need to access some resource storage (i.e. the net) to make a successful identification.
Vector graphics lie somewhere in between... I appreciated the recent contributions about the parameterizations of shapes, I think this will be helpful!
Yes, rendering a graphical representation of (say) a leaf onto the screen would be a good application for vector graphics. However again some additional text describing the image/shape would be useful - for those applications that don't support vector graphics, or indeed visually impaired users.
L.