When apps want to pick an image, they generally don't want to set a list of built-in wallpapers. Currently the wallpaper app only shows up in the list of apps that can provide images if the app that initiates the activity explicitly specifies that it wants a 320x480 image. This won't work well when porting to larger screen sizes. So we should instead just use a flag for this purpose. The wallpaper app should require showWallpaper:true, and the settings and homescreen apps should set that flag when they initiate their pick activities so that the built-in wallpapers will appear as choices.
Also, the current scheme with sizes doesn't actually work as originally intended. Wallpaper doesn't make those parameters required, so if the requestor does not specify a size, the wallpapers will be shown. The email app lists wallpapers as attachments. The contacts app specifies an image size, though, so wallpapers do not get listed in that case.