Searching for images on Google Images is relatively easy.
First follow this link to get to the Google Images Advanced Search page: http://www.google.com/advanced_image_search
The following image shows the Advanced search page. Type in your image keywords and then move to the bottom of the Google Images webpage.
At the bottom of the Google Images search page you will see a field called "usage rights". Using the pull down menu to the right, select an image use policy in order to filter the images the search engine will find for you. Choosing "free to use or share" is highly recommended.