I don't have an iPhone so can't provide a definitive answer - but google suggests:
1. This is a wider problem than just uploading attachments to RIBnet / vBulletin forums - the iPhone Safari browser doesn't support html file upload.
2. That the common work around is to upload to a picture sharing site using an "app" rather than the browser and then include the link on the forum article. e.g.
Flickr (using the
Flickr iPhone app) or the equivalent for
Photobucket.
3. The alternative would require JK to add some 3rd party components to the site (e.g.
http://www.vbulletin.org/forum/showthread.php?t=240652)which would then enable you, via a $2 app to upload pictures (as well as gaining other browser benefits). That might be slightly more likely than with a normal "feature request", because John is a big iPhone fan anyway.