The laws of copyright are quite complex but essentially anything (text, pictures, sound) whether printed, online, or from any other source, is subject to copyright protection. This means for example, that you cannot scan a copyrighted photo from a book and use it on a web page without permission from the copyright holder. For written works, the copyright exists until 70 years after the death of the author. However, even if an original work is out of copyright, a transcription, translation, or photograph of the work may still be subject to copyright.
For some purposes, for example personal, private study use, it is permissible to make copies of copyright material. Use of material in a Ph.D. thesis, however, should generally be viewed as being a form of publication, as a copy of the thesis will probably be deposited in a University library for public access.