As a test, try export the pics from iPhoto (File -> Export) and make sure they are good. If they are then upload them to Snapfish. See if they are still good.
If they export from iPhoto okay, that's iPhoto off the hook. The they appear in Snapfish okay, then that's Snapfish off the hook, and I would then suspect the Snapfish plug-in.
I have a standard wirless AirPort connection through my router.
That describes the connection from your Mac to your router, but tells us nothing about your Internet connection. Dial Up? Broadband? Cable? What speed?
Regards
TD