We recommend that you limit images to those that you are free to "modify, adapt, or build upon." For example, Flickr allows you to search for Creative Commons Licensed content. Here is how to change your settings on flickr so you can only see this content:
If you do use Creative Commons images be sure to read the license requirements and always give proper attribution.
More information on how to cite can be located here. More information on Copyright is in the Copyright LibGuide