By searching with the flickr advanced search, you can find images available for use in papers and presentations. To find images that you are free to use make sure to check the creative commons box at the bottom of the advanced search screen. These images have been tagged with creative commons licenses. The licenses vary in levels of access from sharing to modifying or using the work commercially. All varieties still require you to provide attribution for the original creator and image.
Searching flickr is particularly useful for finding specific shots of architecture, environmental art, or frescoes. Images like these that are available for you to use in your own work can be difficult to find. Searching flickr gives you the benefit of mining the creative output of people who live and travel around the world and document what they find. Because images are loaded by users with differing languages and levels of expertise, be broad in your search strategy to net the largest result set. Consider multiple spellings, broader or narrower categories, and locations for search terms. For example, this image of Casa Pere Milà i Campas (La Pedrera) was found with a search of creative commons images tagged Barcelona.