Question: I want to open a service for people to print portrait images of people they are inspired by. Musicians, scientists, people such as Einstein, Mozart, etc.

What are the legal concerns of using images of **Google Images** and how can I find images which can be legally used for this purpose?

Answer: 
There is no country in the world that has *absolute* freedom of speech. There are many that have extreme limits on it.

The country with the greatest freedom is probably the United States of America but even there there are limits.

For example, it is illegal to defame someone. That is, make a factual statement about a person or organization that is not true and that could damage their reputation.

For your case, as a student of the school you are subject to the rules of the school. If your statement breaks those rules you can be sanctioned. If the school is public, it would generally be as restricted as the government is in limiting free speech but, as stated above, such restrictions depend on where you are.
If you had said it to their faces in the hallway, would you have gotten in trouble? If so, then the answer is yes, you can get in trouble with your school. Probably not with the law, unless you made some credible threat of imminent violence.