Be sure to respect the copyright of these works' creators. Reuse Creative Commons-licensed material under the terms of the license. Public domain works do not have to be attributed unless the creators request attribution (like NASA, for instance). It is always helpful and good practice to attribute works' creators anyways so you remember where you got the work and so that others know it is public domain.
Not sure where an image came from? Try using tineye.com to locate the first use of an image on the web. Sort by oldest.
If you are reusing Creative Commons-licensed images from Flickr, use imagecodr.org to automatically generate a full citation.