Many images that you can find on the Internet are not free. Before using any image, please read the website for usage/rights information, even if this page says the images are free. If you cannot find any, contact the person who created the image for permission to use it. I have done my best to include many free or somewhat-free tools here. The largest owners of photographs, Getty Images and Corbis, sell their photos for money; Buried Treasure is a good article on Corbis and photograph preservation.
Additionally, images with adult content may be found via the search engines and links on this page. Several of the image search engines have adult content filters (you must go to their websites to use them), and Picsearch does this by default.
Most image search engines are keyword-based, using keywords found in the filename or nearby the image or otherwise associated with it. Content-based image retrieval is the science if finding images by the actual content of an image, such as the colours or what objects are shown in the image. There are a number of tools available for this, such as the ‘reverse image search’ links listed above, which allow you to search by uploading an image or using the address of an image already on the web. A Look into Reverse Image Search Tools is an article comparing several. Some websites even let you search for images by drawing or by colour attributes. Idée, the maker of TinEye which is included here has some other similar tools so I have included their labs separately. Both Google and Bing also allow you to find images similar to ones they have already found, and Bing additionally allows finding near-exact copies of images over the web, which is useful for intellectual property issues.
Most images on the web are GIFs, JPEGs (which usually end in .jpg), and PNG. Other formats which you may find include BMP and TIFF. For information about all these and more, see Image file formats.