I am now loading the images from streams. I'll be able to change this to use a file soon. Right now I have a problem: there is no way to get a stream while in a test. Because appx resources aren't available. Or, rather, the Test runner is the app, and it doesn't have any assets. This problem may go away when I get full files. Until then, I will just have failing tests - which explode with COM exceptions. Frabjous day.
All my tests pass again! Refactored enough stuff towards edges that I can instantiate tests and stub off / simulate the annoying dependencies. I could clearly do better than this design, but we've got a starting point.