Difference between picture and source in facebook POST api

I'm new to the facebook posting api's and am trying to clarify the difference between the source and picture fields. They can be found here: Graph API Post

I'm taking a guess here, maybe somebody can clarify. If you have a video, you use source, if you have just an image you use picture? Is there any reason you would use picture AND source in the same post?

Source seems to work for a picture or video...