Todd, i had make same changes like you last week and tried all things (for days) to make it works.
Now, (for my panic), when i changed my png in jar file using your image, MY image (i dont know how) appears, not your image, but mine (the one i customized)!!!
If i click on spark.png inside my spark.jar file (Spark\lib folder), i see your image, but when i get the spark login screen, its showing my image. Is there a cache or something like that who i cant see or find?
Im getting crazy here...
I really want to thanks you, now i can see that is possible to change images, only need to know whats happening on my files.