The work of others that you may want to include in your dissertation (for example images, graphics, archival material or extensive quotations) is likely to be protected by copyright or have specific conditions of use. You should check their status and may need to obtain permission from the copyright holder (who may not be the author). You will need to contact them and explain how you intend to use the material. And of course, material published on the web is usually subject to copyright.
This can be a long and complicated process. No reply does not mean permission to use the material. So start thinking about this early, and consider using open resources if possible.
Large journal or book publishers usually have well-established processes for obtaining permissions which can be found on their websites and are sometimes outsourced to specialist companies. This table summarises permissions guidelines of many academic publishers.
Use your favourite search engine to search for images and sort them by licence type.