When using images of others in your own work, you should pay attention to copyright.
Fair Use rules allow you to quote images, works of art, photographs and short video and audio fragments and reuse these in your paper or thesis, as long as the source is properly indicated. Furthermore, the quote should be functional and can not only be used merely to liven up or embellish your work. For example, for using an image on the cover of your thesis, you will need permission from the copyright owner.
Images without copyright or with a license that allows reuse (e.g. Creative Commons) can always be used for any publication. However, remember to cite the source or copyright owner, also in the case of images with a Creative Commons (CC) license.
In many databases, such as Google Images, you can search for specific licenses or for Open Content. For more information, check our page Finding open content.