Wherever you find your images, be sure to provide proper citations. The University of Cincinnati Libraries provide a useful guide to citing images in an number of citation styles.
Generally, the academic community agrees that use of images in theses and dissertations falls under fair use. Wagner and Kohl's 2011 Visual Resources Association Bulletin article outlines the reasons such works fall under fair use:
Wagner and Kohl also provide guidelines for ensuring that the use of images in theses and dissertations can pass the fair use test:
Although it is generally agreed that the use of images in theses and dissertations falls under fair use, it is best practice to use images that are in the public domain or are licensed for reuse (see Finding Images).
When publishing images in an article or book, you (or your publisher) must receive copyright clearance for any images you want to use that are not in the public domain or are licensed for reuse.
If you are unable to use a public domain or Creative Commons-licensed image in your publication, try to determine where the image came from. Does the work belong to a museum? Check the museum's website for rights information. For images of architecture or public art, try to determine who the photographer is and contact them for permission. Karen Bouchard's Resources for determining copyright and/or public domain status provides a listing of useful sites.
Most twentieth and twenty-first century artists and/or their heirs still own the rights to their works. You or your publisher must obtain permission before publishing anything by them (you may be charged a fee for reuse). These resources can help:
Even if an artist is deceased, and the work of art itself is in the public domain, there may still be issues around the copyright of reproductions. To be safe, you should always check before publication.
Publishers often provide guidelines for including images; be sure to check with each publisher for their specific policies (for example, see the University of Chicago Press guidelines).