To answer this question, first, it is necessary to remember what copyright is. Copyright law is the field that protects the use of the creator’s work once their idea has been physically expressed. For work to be protected by copyright it needs to be both original and tangible. This means that it must the product of your own skill and labor and it must have been expressed in a physical form.
The general rule is that you can print anything on a t-shirt that is under the public domain without infringing copyright laws. There are a number of public domain websites that list images that can be used for personal and commercial uses. Wikimedia Commons is one of the largest public domain image resources. Although most images can be used for commercial use some do have restrictions so read carefully and always double-check. Unsplash offers a range of photography photos that can be used. The British Library has also released 1 million images into the public domain.
You may use
Anything from the public domain Authors who have been dead for over 70 years.
Sayings that are too short to be protected by the copyright law
Your own thoughts and sayings
Anything from a living author
Anything from an author who’s been deceased for less than 70 years
Scripted or literary works of art