Anything in your app package can be referred to using an ms-appx:/// URI (the three /// are important), and opened with StorageFile.GetFromApplicationUriAsync (a static method, see http://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.storagefile.getfilefromapplicationuriasync.aspx).
You can also get your package's StorageFolder object through Windows.ApplicationModel.Package.InstalledLocation, see http://msdn.microsoft.com/en-us/library/windows/apps/windows.applicationmodel.package.installedlocation.aspx. Then you use StorageFolder.GetFileAsync (http://msdn.microsoft.com/en-us/library/windows/apps/windows.storage.storagefolder.getfileasync.aspx) with a relative pathname.