You read that correctly. Microsoft’s Windows 8.1 Photos App by design won’t let you select what images are used on the Photos App Live Tile. Microsoft uses your 50 most recent images, regardless of location, which means they can be on your local PC or on SkyDrive and from the base of 50, randomly selects 5 to use as the Live Tile Slide Show (which changes every 24 hours only).
This was driving me crazy. Here is a semi-ugly work way to specify the pool of 50 images.
1. Pick the 50 images you want in this pool (these must be from folders IN your Library or on SkyDrive). Copy them to a NEW folder on your local computer.
2. Do NOT add this new folder to the Library – it is your backup.
3. Navigate to the images in the original location one at a time in Windows explorer.
4. Make sure the Details pane is displayed.
5. Change the year to something in the future.
6. Click/tap Save.(A progress bar may appear quickly)
7. Do 49 more in the same manner
If you want to see the results right away, follow the steps in http://digitalmediaphile.com/index.php/2013/07/05/how-to-change-photo-tile-live-images-in-the-windows-8-1-preview/