Once you get through the initial submission and pass (i.e you've followed all the guidelines, don't submit inappropriate or copyrighted material, etc.) the design is placed on the website, open to voting by all site visitors. Of course, at this point you can use social media to drive your contacts to Threadless for votes. The designs with the top votes each week are then voted on by the staff at Threadless. This process whittles the selection from several hundred for about 10 new designs which make it onto t-shirts for sale each week.
I never knew about this process and I might give it a whirl. That's where I bought my zombie Audrey Hepburn t-shirt!