It's a great idea, but here's some words of caution...
I have only used my own stock images so far, but checked out a few of the links. Out of 4, 3 had restriction on the commercial usage. For instance this site: http://www.patternhead.com/category/freebies
states the following:
-you can use the downloads for personal or commercial projects
-You may not offer items created with Patternhead files for resale. That includes, but is not limited to scrapbooking, apparel, web design themes, t-shirts, business cards, greetings card, stationery, posters, flyers etc. in both physical and digital formats.
So you can NOT use the designs on Zazzle. And as said... 3 other sites I checked had similar limitations!
Yeah. It started as a list for zazzlers but as web designers and others started using it I started putting links for them in too.
The list itself has now moved and is on a dedicated page on my blog. http://madaussiemusings.blogspot.com/p/free-commercial-use-resources.html
Here is one I did a while back. Not sure if it is what you are after.
Also this is a tee from another zazzler that I always liked
BTW - A BIG thankyou for featuring one of my tees on your Death by Metal blog. Gave it a shoutout on twitter for you.
courtrhodes - watch this vid. It will explain how to post linked images here.
Anyway back on topic - this is my latest offering.
Not sure if this is heavy enough for you