To answer your question, yes the templates are loaded and parsed at runtime. We don't have any sort of automated support for automatically packaging up image references into GWT resource bundles.
It's certainly a compelling feature which we could look at supporting.
The alternative would be to use GWT resource bundles as normal and inject them into the template. But that certainly loses the appeal of designers being able to work with them as you say.
In any case, I could look at prototyping a way to generate bundles by parsing the templates at compile time.
Automatic bundling of images would be awesome. That would make the templates much more convenient.
An interim measure would be some way to determine which image files don't need to be downloaded because they are being replaced in the code. Perhaps another attribute in the HTML file?
Thanks for the explanation, Mike.
Just wanted to add my voice to this - I also think this would be a great feature.