Ashish did a great job with his demystifying URIs.. post.
I spent a bit of time discussing this topic with him last month due to a regression bug ( fixed before beta1 shipped).. During my 'discovery' I wrote a little test application that summarized it (and tested it all content resolving mechanisms ) ..
What this application does is include 3 images, packaged all possible ways (resource,content,embedded), and shows you how to load them from XAML.
If you pay attention, you will see the sample also loads "external" assemblies NOT in the XAP and it shows you how to cross reference content using the /assembly;component syntax .. which Ashish forgot to mention..
I am going to leave that topic of loading external assemblies for a later post (in hopes ashish tackles it in detail like he has been doing); if you are itching to try it, the source is very small and very easy to follow...
Sorry that my test apps are so ugly. If you can recommend a book (or web reference) on how not to offend the world with ugly colors but at the same time do it with zero overhead to my coding, please send me a link.