If you use an image you did not create, you must attribute the creator. This is required whether you had to obtain permission or it was freely available to use. Either way it is necessary to give credit to the creator or copyright holder. Be sure to read the usage rights statements on websites to understand if there is specific wording that needs to be used.
Also, make sure you are using the correct citation style for your discipline, as the requirements will differ slightly. In general, there are potentially three places you will need to provide citation information: