In general, you always want to cite sources--this includes images. If available, include the name of the creator and the source of the image. If you need copyright permission, do not hesitate to ask a friendly librarian for help!
Creative Commons Images: Include the title, creator name, source of the image (URL), any copyright or license information.
Websites: check if a website has a term of use (or copyright/permissions) and see if you are allowed to use images. If you cannot find anything, then use the "creator, title, source" as explained above.
Please note: these are general guidelines for using reusing images (figures, tables, photographs, etc.). See the style manual for your chosen citation style for more complete information.
Do I have to give attribution? YES!
Do I have to obtain copyright permission? MAYBE...
You must give credit, or attribution, to the creator of an image just as you must give credit to the author of text. By doing so, you avoid plagiarizing (i.e. claiming the image as your own). In your bibliography (or list of sources used) you generally reference the source of the image, not the image itself.
Many images have copyright protection (even if you do not see the copyright symbol!). Under the provision of "Fair Use", you can generally reuse images for a class presentation (including poster projects and other kinds of visual presentations), a classroom session, or a paper/thesis. Once you publish your work (e.g. post your poster on your blog or publish your paper in a journal), the situation becomes questionable. Unless the image is in the public domain or under a creative commons license, you may need to obtain permission from the copyright holder to use the image.
This is only the tip of the iceberg as far as copyright goes! If you have specific questions, please contact your subject librarian.
All images require captions! At a minimum, include a figure number and title. In most cases, include a statement such as "Reproduced from reference #n". Include copyright information if copyright permission was obtained or there is a creative commons license.
Preparing Your Chapter (Manuscript) for ACS books:
Citing Your Sources (ACS Style) from UC San Diego: https://ucsd.libguides.com/chem6c/acs
Referencing Figures, Tables & Images from Edith Cowan University: https://ecu.au.libguides.com/referencing/figures-tables-images
Documenting and Citing Images from University of Southern California: http://libguides.usc.edu/c.php?g=235130&p=1560459