Added more examples to possible sources of image resources

This issue was discussed a few months ago [on the mailing list](http://lists.w3.org/Archives/Public/public-webappsec/2013Apr/0065.html), and it was [decided to wait for better hooks](http://lists.w3.org/Archives/Public/public-webappsec/2013Apr/0071.html) in order to make a generic way to define an image resource, and avoiding the need to enumerate all of them in the spec.

While that's still a good plan, I believe there's no harm in making the current list of possible image sources slightly more complete in the mean time.