Just saw this thread and wanted to comment - pardon if this has already been addressed.
You can 100% use any theme image using an HTTP location over the PKG. In fact, we never use the pkg, just in case we want to change something for quick testing. I think the only images that have to come from the package are images defined in the manifest. If you're not seeing an update quickly, and to ensure you're not caching, just through a param at the end of the image (
http://yoururl.com/images/yourimage.pngv?1234). We use a timestamp to ensure a unique value each time. This is not always best practice since loading fresh each time isn't the right thing to do unless you expect these images to change frequently. We'll load a unique image then store it in cache, but for channels that are using dynamic images, we'll always need to pull a fresh one when the users comes in from the top. Also, if your using a CDN, the time to propagate varies so that combined with caching on the box could prevent you from seeing any differences. Hope you got it all figured out.
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Float Left Interactive Team
www.floatleftinteractive.com