The "World Peace" graphic you linked to was created by someone.
Whether it was by someone who placed their work in the public domain for free use to anyone, or whether it requires a license from the creator of the graphic prior to using it, is a question mark. However, unless there is a clear indication that the copyright holder is giving up his/her rights in the graphic for free use, you should assume that the work is rights-protected and requires permission from the copyright holder to use.
To use this or other graphics on tee-shirts without permission from the creators of the works would be copyright infringement and could lead to a cease and desist letter, a settlement demand notice for X amount of dollars, and/or an infringement suit.
In other words, if you did not create a work, it is (generally) not yours to freely use. You must locate the copyright holder and get permission.