In the United States, any creative work - including images, music, and artwork - is copyrighted from the moment it is completed (17 USC, Section 102). Similar laws apply to websites hosted in other countries, so you should assume that every image, word, sound, and piece of software you find on the Internet is under copyright.
How this applies to you: As the creator of a thesis, if you wish to include other people's words, artwork, or images in your own work, you must seek permission as appropriate from the copyright holder. In addition, you should always cite your source.
If you are seeking images for your thesis, it may be useful to know that - in some cases - content creators allow more liberal reuse of their work. These creators specify how their work can be reused using Creative Commons (CC) licenses. Creative Commons can be a useful source of images for your thesis, but please observe the creator's preferences when using this content, which is searchable on the Creative Commons website.
This Ars Technica post from 2011 is a good general guide to the use of Creative Commons images: Creative Commons images and you: a quick guide for image users. You can also read more about the Creative Commons here and here.