Images you find online will probably be subject to copyright laws and require permission and/or attribution to use. Some images may be licensed for use through Creative Commons.
More information on Creative Commons licenses is available at creativecommons.org/.
For more copyright guidance please see www.hw.ac.uk/uk/services/is/learning-teaching/copyright.htm
How you cite and reference an image in your work will depend on which citation style you use.
Harvard and IEEE: Check Cite Them Right Online
SIAM: Check the SIAM referencing guidelines on the citing and referencing guide