Searching for Images on the Web
Search for images that are free to use without the need to obtain a specific statement of permission.
Read here about reusing content outside of Wikimedia.
Flickr Creative Commons Advanced Search
Limit your search to those that have a Creative Commons license (at the bottom of the page).
These images have already been designated as available for download and use by the owners, but each has a different license with different usage requirements. Check out the breakdown of licenses via Creative Commons on Flickr. Images on Flickr that do not have a Creative Commons license will have an 'All Rights Reserved' copyright attribution. In this case, you should contact the owner before use, but in time sensitive cases, one can look to Fair Use for guidance for usage.
Search for images available from reputable institutions via Flickr. These images are labeled as having 'no known license'. Most images here are scanned photographs, lantern slides and posters.
Limit by size by choosing 'Larger than 1024x768' to find higher quality images; limiting by an even greater size (such as 2 or 4 MP) increases your chance of finding a high quality image
Limit by Usage Rights and choose 'free to use or share' (this is a more reliable way to find images which you have permission to use and modify), which is the last option on the page.
Creative Commons Search Portal
Use this portal to that searches other places on the web using their search engine to identify content that may be labelled for reuse
Always identify that a resources is actually labelled for reuse.