Works of others you can wish to include in your thesis (for instance images, graphics, archival material or extensive quotations) are probably protected by copyright or have certain conditions of use. You should check their status, and you may have to obtain permission from the copyright holder (who is not necessarily the author). You must contact them and explain how you will use the material. And of course, materials published on the Web are generally copyrighted.
This can be a long and complicated process. No answer does not mean authorisation to use the material. So start thinking about this early, and consider using open resources, if possible.
Large journals or books publishers generally have well-established procedures for reuse permissions which can be found on their websites and are sometimes outsourced to specialized companies. This table summarizes permissions guidelines for many academic publishers.
In your favorite search engine, you can search for images and sort them by license type.