The policy I have developed over the past week is this...
1. If the thumbnail is fetched from the website I'm linking to, I put the onus on THAT website to properly source the image.
2. If I don't like any of the images that the snap fetches automatically, and I use another website to provide an image (like my spaceport snap earlier), I put the website I got it from as the source.
3. If I created the image, I don't put a source.