You need to assume that every image you come across is protected by copyright unless a) you took the photograph or drew the artwork yourself, or b) the work is so old that copyright has now expired. In the UK (and many other countries) the copyright term is the lifetime the author of the work plus 70 after their death. This means the copyright term in many cases could be around 120 years from when the work was first made.
However there are instances in which the author of a work is happy for others to freely use their work and often this is done by applying a Creative Commons licence to the work before making it available to the public, especially via the internet. But beware, there are several different types of CC licence and some explicitly exclude commercial use, so that would rule out you using them on tee shirts which you sell. Therefore before you start looking, I suggest you become familiar with the Creative Commons rules here
and what the various symbols mean.
You can search for CC licensed images via the Creative Commons own website
(make sure you click on the radio button that says 'use commercially') or use a search engine such as Google using the term Creative Commons along with a particular subject you are interested in.
Once you find something you wish to use, make detailed notes about where you found it, the author's name and the licence which was attached to it. You may need to rely on such information if, later on, you are challenged about your use of that particular image. If you are in any doubt about the validity of a CC licence, contact the person who claims to be the author to verify the true status of their work.