Thanks for the suggestion, but sorry to say it does not work correctly. I set my real wallpaper to be one of the included apple wallpapers that iOS4 came with. When I added your code, I do see the default wallpaper load, but I can also see WeatherFurik's theme wallpaper right underneath it in a strip to the left. The apple wallpaper on top of it is also low res.
I'm now convinced that, as Krafty said, each weather widget needs to have an included wallpaper to work. These widgets are probably instructing iOS to use their wallpaper, which they've coded to have javascript weather in it.
I'm fine with loading wallpapers into this theme folder from now on. But there has to be a way to load retina wallpapers. Each time I change some of the coded height and width pixel sizes in these js and html files, it breaks the widget. When I respring, I either won't see the weather and icons, or I won't see the date and time, or I won't see anything at all!