Unfortunately, I don't think there is one comprehensive source of info for that. I created the Simdex, which is a guide to the Sims 3 Store content, but even that doesn't include the wallpapers that came with the worlds yet. It is on my to-do list, however, so hopefully that will happen soon.
Your best bet is to either create a new Sims 3 documents folder and install the worlds one at a time OR piece together an idea of the wallpapers by looking through reviews—I believe AznSensei shows pictures of some wallpapers in his reviews, but he doesn't provide a complete list. He says there are 29 wallpapers with Midnight Hollow and another 7 with the Toy Store venue, but only shows the venue ones. I can't find anything on the wallpapers for Dragon Valley or Hidden Springs.
Sorry I couldn't be more help. :(
