Hi Eric, I'm not a lawyer and as a business, it would be best to contact a copyright lawyer to get yourself onto firmer ground.
This is what I know about creative commons:
0. All works without a creative commons declaration should be considered All Rights Reserved, Don't use them!
1. All works with the creative commons non-commercial (CC-BY-NC) should not be used.
2. All creative commons licenses except for CC0/PD REQUIRE you to attribute the original author/creator. This means that your t-shirts must include the copyright, that they are CC images and even a link to the place where they can be found. If you don't attribute correctly, then you loose the right to use the artwork completely. These should be on the shirt and not just on the website.
3. Do tell the authors. While it's not a legal obligation (except for some jurisdictions like Germany) morally it's good to let artists know and if they want you to remove the work or engaged in profit sharing, that's something you can consider as a moral obligation. It would be good of you to reward and supply resources to your suppliers as it will lead to future artworks.
4. Use the highest quality work you can, no one likes their work to be badly reproduced as a low resolution jpeg.
Good luck and DO check out a lawyer, it's super important that you get proper advice as I'm only some guy on the internet with vague memories about how things work.