As previously noted, using an image you find online can be problematic. To locate images using Google that are cleared for legal re-use, follow these steps:
Visit images.google.com and look for the Settings option, usually in the lower right hand corner, then select Advanced Search.
Type in your topic:
Then go to the bottom to narrow by the types of usage rights:
Select which option works for you, then click Advanced Search.
You should still cite these images, but you do not need to get permission to use them.