"Can I use that picture?"
Image credit: This handy chart is from a Lifehacker article written by staff writer Patrick Allan.
This chart's text is small! So, hold the Control key down and press the + key to zoom in and the - key to zoom back out.
Flickr
Flickr is an image-searching site that lets you search by keyword - and then refine your results to make sure that you are looking at Creative Commons licensed images.
Google Advanced Image Search
The advanced Google images search lets you (all the way down at the bottom under usage rights) choose that you want to search through images that are allowed to be produced/reproduced.