You may find a lot of irrelevant images, because the web search engines use your search terms to search for images.
They search for your terms in the filename of the image, the link text pointing to the image or text adjacent to the image.
Essentially they make a guess that the words are related to the image.
Web search engines cannot grant you any rights to use images for any purpose other than viewing them on the web. Therefore if you would like to use any images found through search engines, you must contact the site owner to obtain the requisite permissions.
Image search engines cannot make adequate assessments of the aesthetic quality of images they index or an image's suitability for your particular purpose.
Web search engines are unable to index images held within databases. For finding high-quality images suitable for research and presentation in your discipline please refer to Image Resources by Subject or look within discipline-specific guides.