heya,
I don' know for sure if I'm right, but I think you can change the settings in zune. The thing is: i'm not sure if the changes will effect syncing from zun to the phone or just deal with pics being synced from the phone to the pc...
Anyways...Just go "settings", then "phone" and choose "pictures and videos". There you should have the option to choose the quality. (I had to translate the setting-options from german to english so they might vary a bit)
Hope I could help