What content is protected by copyright?

Images, audios, texts, graphics, diagrams and the like are protected if they reach the level of a work. The concept of "work" presupposes a personal intellectual achievement, an achievement with an individual character that stands out from the everyday, commonplace or usually produced.
A work is protected by copyright as soon as it is created. Even if no indications such as "copyright", "all rights reserved" or a © are attached, you are better off assuming that copyright protection exists.

What copyright restrictions apply in schools and universities?

Content that does not reach the level of a work. In the world of flashcards, this includes, for example, foreign language vocabulary as it is used in everyday life. Beware: Although the vocabulary itself is not affected by copyright, a specific collection of it could be classified as copyrightable by case law due to its selection, arrangement and classification.

Scientific findings. Beware: Although scientific findings per se are not affected by copyright, their specific presentation is subject to copyright protection. Therefore, you may only quote scientific works in extracts at most.

Content that has been released 100% into the public domain under a CC0 license. CC stands for the "Creative Commons" license system and 0 means that the creator waives all rights. See also the section below "What are Creative Commons licenses?"

Works whose creators died more than 70 years ago.

Official works such as legal texts, court rulings or regulations issued by authorities.

Limitations on copyright in an educational environment

Students and teachers at (higher) education institutions are privileged under copyright law. They are permitted to distribute copyright-protected content without consent...
if only parts of works are used,
the use relates to teaching,
and the circle of addressees is limited to the students and teachers involved.
Specifically, this means for example that you are not infringing any copyrights if you, as a student or teacher, upload an image from a copyright-protected school book to a private set of flashcards that you share with your class.

What is the right to one's own image?

The right to one's own image is a protective right related to copyright law that prohibits you from publishing or distributing images of clearly identifiable persons without their consent. There are some exceptions to the right to one's own image. However, if you stick to the basic rule, you will always be on the safe side.