How can I force the Pictures app to refresh its library?

This can happen if Windows Search hasn't had a chance to index the photos yet, or the index is corrupt. Rebuilding the index can force the pictures app to rebuild its own index.

to rebuild the index, or ensure the relevant folders are included from the Start screen, type Indexing Options and choose the option of that name that appears under the "settings" section.

If the option doesn't appear, it may be due to the search service having been disabled (for example, to install Outlook 2013 on x64), so it would need to be reinstalled from control panel.

I had the same issue and was able to fix it by following the instructions in the first post in this thread: I closed the Windows Photos App, then navigated to the folder

%localappdata%\Packages\Microsoft.Windows.Photos_8wekyb3d8bbwe\LocalState

and deleted its contents. When I started the Photos App again, everything was OK.