And someone has just asked the same question on iS's forum, so you might like to check in to see if there's an official answer:
http://www.istockphoto.com/forum_messages.php?threadid=359544&page=1BTW, as an example of how a file's keyword order changes on acceptance, here's one I uploaded last night, was accepted today and isn't showing in my port/searchable yet:
Keywords in the order I uploaded them:Siena, Italy, Basilica, Church, Santa Maria Dei Servi, Middle Ages, City, Urban Scene, Tuscany, Europe, Cityscape, Roof, Elevated View, Unesco World Heritage Site, Place of Interest, Tourism, Medieval, History, House, Architecture, Local Landmark, National Landmark, Southern Europe, Colour, Photography, Horizontal, No People
Keywords in the order they now show, with obviously no views, sales etc:Siena, Italy, Basilica, Church, Tourism, Tuscany, Southern Europe, Europe, Roof, House, Cityscape, City, Santa Maria Dei Servi, High Angle View, Horizontal, Urban Scene, Photography, No People, Middle Ages, Color Image, Medieval, History, Local Landmark, National Landmark, Famous Place, Architecture, UNESCO World Heritage Site
So why they're asking us to put the most important words first when they're going to hange them anyway is another mystery.
Who knows?
And yes, I'm going to change 'tourism' to 'travel destination' - which was a previous mapping for the more usual, in the UK, 'tourist destination'.