Each writing style has specific instructions regarding how to cite images. Check the handbook for each style for info. Remember that artwork is also information and a creative product, so "authorship" needs to be noted when you use someone else's work.
Even if an image has a Creative Commons license that allows you to use it, make sure you read the details of the license and follow those directions.
When conducting research, the same rules apply to images in terms of plagiarism and copyright. Always be aware of where you found your image (or text). Cite it and/or attribute it.