The image number must match the key. (Yes, it's redundant, just do it.) The
alttext is an optional string describing the image (for non-graphical
displays and visually impaired users.) The width and height are the image
size -- these fields are required! And the url should be a URL from which
the image resource can be loaded. This may be an absolute URL, or relative
to play.html.
This information is not yet generated by Inform. If your game contains
graphics, you can manually edit this file to include all the image
data. Alternatively, use the "giload" command of blorbtool.py: